Hi Holger,
For your information.
in upgrade first we need to upgrade 3.x to 7.0 and then to 7.4.
directly we can't upgrade form 3.x to 7.4.
Coming to CO-PA - Try to upgrade your copa data flow from 3.x to 7.0- check the loading functionality.
if its works fine then no issue. As my guess migrate data flow will work as expected way.
Thanks