No problem, Jochen. You can do as I proposed and when you feel like you have decided for one or the other, you can do one final sync for all your devices (so everything has got the exact same data) and then disconnect the platform(s) you’re no longer using.
That doesn’t work with backup.
I have install OF3 on iMac.
I open the OF2 backup database with OF3.
I see OF3 with the actions from OF2.
I quit OF3.
I start OF3.
There are no action from OF2 inside.
If you open a backup file, it will not replace your database. It will just be displayed so you can look at it and maybe copy some (lost) data over to your actual database.
If you are already synching, I strongly suggest you tell OF3 to sync as well, so it pulls the database from the server. Afterwards, everything will have the same data: OF2, OF3 and your iPad.
If you don’t want to sync, it is probably a good idea to ask support for instructions on how to move your data over.