I would copy all messages and folders to a new folder under the special Local Folders account. The Local Folders account in Tbird is special because it does not connect to any remote mail server via the internet, so it's the ideal account for keeping a local archive of your messages. If you have a lot of folders under your POP account, the fastest and easiest way would be as follows:

1. Take Tbird Offline via File > Offline > Work Offline.
   
   
2. Compact folders via File > Compact Folders.
   
   
3. Create a new folder under the Local Folders account. I'll refer to it as Old Provider in this tutorial. Give it a short meaningful name. This is where all your old provider's messages and your folders will be stored going forward.
   
   
4. Create a subfolder under the Old Provider folder and call it Temp. This is just a temporary placeholder; it will be deleted later on.
   
   
5. Open your profile folder via Help > Troubleshooting Information > Open Folder button in the "Profile Folder" row.
   
   
6. Quit Thunderbird via File > Exit and make sure it's not running in the background. This is important.
   
   
7. In your profile folder, open the Mail > {pop server name} subfolder. Inside you will find mbox files (they have no extension) and folders ending with .sbd. Select ALL the mbox files and sbd folders and copy them. I find it easier to change the view to Details view then sort them by Type. Those are the only items you need, they contain all your messages and folders/subfolders.
   
   
8. Move up/back one level to the Mail folder and open the Local Folders > Old Provider.sbd subfolder. Inside you'll find two files named "Temp". They are files linked to the Temp folder created in step 4. Delete them both, then paste the copied items into the now empty Old Provider.sbd folder.
   
   
9. Launch Thunderbird and look under the Local Folders account > Old Provider for your messages and folders.
   
   

If the old provider's account is still receiving messages, simply copy or move them into their respective folders under the Local Folders account, to make sure your local archive is up-to-date, until such a time when the old account is no longer reachable, then you can delete the old account from Thunderbird and only keep the new one.

This is the simplest, fastest and cleanest action plan I can think of. An alternative plan is to keep the old account in Tbird and rename it, change its server settings to work with the new provider and delete the old provider's saved passwords, then provide the new provider's login credentials when Tbird connects to the new servers. I don't recommend this approach because it is a "dirty" one, so to speak, but that's just more of a personal preference to minimize mixups down the road.

Stans, thank you very much for the detailed instructions. On first glance it looks complicated but after a closer look, I believe I can get it done. I do have a few questions. 1) I assume the new provider(NP) will get new Inbox, Drafts,etc when I register. Will my old folders be visible and able to be used in the fashion they are now to store mail from the NP or will I have to create a new series of folders? 2) I assume I wait until after all the work you describe has been done before I register the NP. 3) The 'dirty' alternative is what I thought of from a logic point of view. Would you elaborate on what you mean by 'mixups down the road'? 4) Regarding your Item 7 - I'm a bit confused about the outcome. Isn't a copy and paste going to leave me with 2 copies of my folders? Is this a problem or have I misunderstood? 5) Is this approach you have described basically the 'global inbox' approach?

1) I assume the new provider(NP) will get new Inbox, Drafts,etc when I register.

If by "register" you mean adding it to Thunderbird, then yes.

Will my old folders be visible and able to be used in the fashion they are now to store mail from the NP or will I have to create a new series of folders?

Yes they will, if you opt for the "dirty" option instead of adding the NP account from scratch.

2) I assume I wait until after all the work you describe has been done before I register the NP.

If you want to keep it simple (avoid mixups/distraction when following those instructions), then yes.

3) The 'dirty' alternative is what I thought of from a logic point of view. Would you elaborate on what you mean by 'mixups down the road'?

The dirty alternative involves modifying some of the account information that Thunderbird already has on record for the old provider's account, so if or when something breaks later and you're trying to fix it with or without someone's help, you may encounter references to the old non-defunct account which may add to your confusion/difficulty in following through with the troubleshooting, especially if you forget that your new account is reusing some of the old account's records. An example, is when you need to work with the contents of the Mail > {server name} folder, like in the steps I posted. You may get surprised to find the folder named after your old defunct account and be tempted to "clean up" after the old account, only to realize the new account has been reusing that folder and is now broken following the cleanup exercise. This is just one possible scenario that could trigger a series of mistakes.

4) Regarding your Item 7 - I'm a bit confused about the outcome. Isn't a copy and paste going to leave me with 2 copies of my folders? Is this a problem or have I misunderstood?

The outcome is to eventually delete the old account once it ceases to exist and retain the new one only, with all of the old account's messages and folders archived under the offline Local Folders account. You'll have two copies during this transition, unless you MOVE instead of COPY them. I like playing safe, so I prefer to copy first, check things are fine, before deleting the original copy. Not long ago we had a thread in which files mysteriously disappeared (seemingly for good) when the user tried to MOVE them to a different drive. To date, they haven't been found in the source or destination locations! Years of the ONLY copy of messages were lost this way, with no way out of that mess!

5) Is this approach you have described basically the 'global inbox' approach?

Nope. That approach involves eventually getting rid of the old account, leaving only the new account in place, with its own separate inbox, while the messages of the old account get archived in a separate local folder. Global Inbox requires at least two active online accounts that fetch emails and store them in one Inbox.

Thanks Stans for your update. As much as I would like to take the easy route I see from your comments it could easily become more of a disaster than anything else. I'll leave this open for now and post my results, or questions if need be, when all is done. Thanks again.

Whichever approach you choose, just make sure you take a backup copy of your entire Thunderbird profile folder BEFOREHAND while Thunderbird is NOT running. Simple copy and paste into a convenient location will do. I also recommend regularly doing so, because bad things can happen for any reason and a backup remains the best recovery strategy for such situations. With a backup in place, you don't even have to worry about an approach going awry.

The copy is made and I'm looking at Item #7. I have only one .sbd and that is associated with Trash. The rest are type FILE without an extension or type MSF FILE with .msf extension. Is this something for me to be concerned about?

The copy is made and I'm looking at Item #7. I have only one .sbd and that is associated with Trash. The rest are type FILE without an extension or type MSF FILE with .msf extension. Is this something for me to be concerned about?

No, it's all good. For each top-level folder you create in Thunderbird, an mbox and msf file is created. If a folder contains a subfolder in Tbird, then a folder with the .sbd suffix is created, inside which an mbox and msf file are created for the subfolder. In your case, it seems Trash is the only folder that contains a subfolder, hence the only .sbd folder in that account's storage location.

The copy is now happening with the all the FILE and MSF FILE going into OLD PROVIDER.SBD. I see there are also an OLD PROVIDER FILE and OLD PROVIDER MSF FILE at this location. Is there a reason why I did not copy the files to these respective files instead of the OP.SBD?

You can't copy an mbox file into another mbox file, but you can copy an mbox file into a folder. That's why. You don't need to copy the msf files. I suggest you delete the msf files in the destination folder after the copying has finished, so that Tbird can index the messages in their new location. The msf files are index files for the mbox files.

Everything appeared to have come out as planned. The Local Folders had the Old Provider folders. I was able to access and add new entries as they came in on the account. After I read your last message I deleted all the MSF files. I looked at the OP FILE and the OP MSF FILE that I mentioned. They appeared to be empty. I deleted both of them. I don't know if it was due to that deletion but I no longer show any of the copied folders in the Local Folders section. I checked and the physical folders exist, they just don't show as being connected to anything. Should I start the process all over again?

You shouldn't have deleted the FILE type file. Remember I said you NEED the mbox files (the files without extensions, of type "FILE"). The msf ones you don't need to copy. Read my closing statement in step 7 again. Restore the deleted mbox file.

Sorry Stans. I don't see a way to " Restore the deleted mbox file.". The OP FILE and OP MSF FILE were in the same folder as OP.SBD which has all of the mbox entries I copied from my current account. That OP.SBD is still there, it does not show up under the LOCAL FOLDERS display. I do not know where the OP FILE and OP MSF FILE came from.

Deleted files should be in the Recycle Bin. They were created when you created the OP folder in Tbird. Whenever you create a folder, an mbox and msf files are created for it. If you create a subfolder in it, an sbd is created in addition to the mbox and msf files. If you emptied the bin, start from scratch.

I started from scratch since I did not have the deleted files. Upon completion and after I launched TB to check things out, I deleted the .MSF files for OP and TRASH. I also copied the .SBD for TRASH into the new file. Everything looks to be OK. Thanks very much for your time, effort and patience!