Just to clarify, a profile is all the accounts that were defined in it. If you have a profile with accounts X and Y, and then import a profile with account Z Thunderbird switches from the profile with accounts X and Y and now has the profile with account Z. If you have concerns, my suggestion is to copy the current profile elsewhere (when thunderbird isn't running) to give you some security.

Thank you, david. I recently needed to replace a 3 year old machine after the latest OS Sonoma update burned out my fans. (I have very low bandwidth and slow downloads and it took 4 hours) Since July 30 I had only been able to check several accounts on the old machine that was replaced and was anxious to get those accounts set up on my new machine. I was concerned it might overwrite the oldest account but it appears to have imported without overwriting/replacing all the downloaded messages.

It is working fine now. If only I had tried earlier, I might have seen the note in the TB interface where it told me it would not overwrite any files. Yes, I saved a copy of the existing profile with only that one account but I have not needed to use it. Good suggestion.