After it turned out that a Pi3 isn’t a suitable platform for HAOS, I managed to score a Pi4-8G which should arrive any day. I have the whole system running from SSD, no memcards involved. Is the migration as simple as plugging the SSD from the Pi3 to the Pi4 or are there things to pay attention to?
No.
There are two separate images for the Pi3 and Pi4. You will need to create a backup and copy it off the Pi3. Then install a fresh Pi4 image on the Pi4. Then restore the backup on the Pi4 (during onboarding).
One other thing to be aware of is that your database will not backup without corruption unless you stop it before creating the backup. This is only an issue if you care about your long tern statistics. The other way to do this is to copy your database to the new system when Home Assistant is not running on the Pi3.