When moving the drive that has OpenBSD installed on it to a new machine, the new hardware is not automatically configured.
For example, the new network interface does not appear in ifconfig, and the new DVD drive does not magically appear in fstab.
Surely there are other adjustments that would need to be made as well.
Therefore,
What changes need to be made after moving the system drive to a new computer?
For OpenBSD, as long as the computers have the same CPUs and you are using the standard kernel, I doubt any changes are needed. I have done this many times without issues.
But, at the very least you should do a fw_update