O O Thunderbird 3.0 - Novos recursos e alterações página tem algumas informações sobre isso:
While it is not recommended, you can use the same profile with both Thunderbird 2.0.0.x and the latest nightly build for Shredder (i.e Tb3), and switch back and forth between versions.
Algumas ressalvas:
The main impact is that not all add-ons work with both versions [...] and that your cookies and passwords are stored in different files in Shredder, so if you reverted you'd use the (old) Thunderbird 2 versions of those files.