1. As the account name will change, create a new account - but keep the old one
2. Since this is a POP account, you will not lose any messages when the old account dies, as all messages are on PC
3. the filers are in a file called msgfilterrules.dat. it is a simple text file, so you can edit it with notepad. whether they work or not will depend on careful editing to adjust text from old account name to new name. Often, a simple find/replace works. Anyway, with a minimum of work, this is doable. After tweaking it, COPY to the new account. To do that, click help>troubleshootinginformation, scroll to 'profile folder', click 'open folder', click the Mail folder and then the folder for old POP account, copy the msgfilterrules.dat file and then select the folder for the new POP account, locate its default msgfilterrules.dat file and overlay it.
4. to 'clean up, I suggest copying the message folders from old account to Local Folders so that you don't lose them. A simple way for that is to click help>troubleshootinginformation, scroll again to 'profile folder' and click 'open folder' and, again, click the Mail folder and then the old POP folder. This time, look for message files, which will be in twos: e.g., inbox, inbox.msf, sent, sent.msf.. Ignore the msf files, but COPY the sister files and paste into the Mail\Local Folders file (rename if needed to prevent overlaying any existing files).
5. once done, check that you salvaged all the messages and you can then delete the old account.

When done, be sure to backup everything. Here is a sample of backup and restore steps, if ever needed. BACKUP

1. ensure thunderbird is not running
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media

RESTORE

1. ensure thunderbird is installed and NOT running
2. enter %appdata% in search window on task bar and press Enter key
3. delete the thunderbird folder
4. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there.
5. start thunderbird and all should be there

Thanks David. I'll give it a try and let you know how it works out.