Ok, I figured it out.
I have copied ~/Library/Preferences/com.devon-technologies.think3.plist over com.devon-technologies.think.plist in the same folder.
Seems to work fine, I hope I havn’t broken anything.
Maybe you could provide a function to export/import settings in the future.
I’t would be helpful to transfer your preferences between installations, too, and also as a backup before fiddling with different options.
The most important settings are migrated, e.g. sync, imprinter configurations or label colors. But as the bundle identifier and application name will not be changed anymore, this is the last migration of this kind anyway.
DT4 Server seemed to import most of my settings, but the Server settings did not come over. Had to re-enable start on launch, change the Bonjour name, and recreate users and permissions.