Well, when you have a classic DB and you install Mk II, it will convert your DB for Mk II. It only converts from classic, though. It won't convert from Goals Edition. So if you can convert your Goals Edition DB back to a classic DB, then it would theoretically be able to convert to Mk II after that.
It's just speculation, though. I haven't tried it or anything. But if you were to attempt it, I would do whatever you can to make backups for every step of the way. Nothing involving any version of HITDB ever seems to be easy.