As per the FAQ they seem to share the exact same profile so you can seamlessly switch between them (provided both are the same dot release).
Edit: in case that doesn’t work, run betterbird -p
and select the right profile. For me it chose the wrong one.
I just gave it a try and it does not pick up my existing Thunderbird data automatically, or prompt to import it.
The instructions change if you’re using flatpak. I was only able to get it to work with a manual import