Are you sure about that? Could you explain why not?
It's no different to flashing a rom cooked from another cid. Of course jelly bean won't boot on older hboots and vice versa, but that's the only limitation as far as I can see.
Infact, I have cid HTC_001 and hboot 1.28, I'll try the backup for 621 you have listed for a scientific test.