Opnsense has been far more consistent with their upgrade releases.

Migration is really a complete reinstall and rebuild of your rules. I took a backup of my physical pfsense box and restored it to a VM then reinstalled opnsense on the physical hardware. With the pf box running in a vm I could just reference it while creating new rules.

I think someone made a script to load a pfsense backup onto an opnsense instance but I really think you’re better off with a clean slate.