I have an SD card which works fine on a Pi 3B, with network config on it (it runs a wireless AP and has DHCP setup on the wired port).
I move it to a new 3B+, and I see that it doesn't come up on the network (no wireless AP appears and no ethernet lights come on on the wired port).
I'm guessing this is probably expected given that the 3B+ changed out all the network hardware.
But is it possible to make the migration work by (say) installing all the drivers for the 3B+ hardware while the card is in the 3B?