I suggest NOT doing that. Instead, do not use the new computer until you do the cleanup. This avoids many problems. Once you have what you want, make the transition in one step:

I encourage updating old PC to the version you will install on new PC before the move. Not required, but makes life much simpler.

Once you're OK, here are steps: Here is the link for moving a profile in Windows when installed in default location:

https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

here is the info in brief steps:

1. ensure thunderbird is NOT running on old machine
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
5. ensure thunderbird is installed on second computer and NOT running
6. enter %appdata% in search window on task bar and press Enter key
7. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it
8. start thunderbird and all should be there

The above steps transfer everything - accounts, addressbooks, settings

Thankyou for this- but I really do want to 'start again' and keep ( the last 20 years) my current data frozen, but with access for anything I might need. Maybe an alternative is to export my Thunderbird contents to another mail facility ( eg gmail or yahoo) and then start anew on the new PC Anyone got any suggestions?

Thankyou for this- but I really do want to 'start again' and keep ( the last 20 years) my current data frozen, but with access for anything I might need.

Where are those 20 years of messages stored in the Thunderbird profile of your old PC? In POP accounts and/or in your Local Folders? In a registry outside the profile folder?

Maybe an alternative is to export my Thunderbird contents to another mail facility

Maybe MailStore Home https://www.mailstore.com/en/ ?