"upgrade offer you can get on a card you've gotten SUB"
Not sure I understand what you mean...let's say you get a card with a SUB (say the Hilton Surpass). You want to upgrade that card to the Aspire a year or two later- do you get a second SUB when you do that?
Similarly, if you have an Aspire where you originally got a SUB, then downgrade to either a Surpass or even a No-Fee....then a few years later want to upgrade back to an Aspire, do you get a sign up bonus for that?
I guess I don't understand the purpose of upgrading/downgrading...why not just apply for a new card, with a SUB, then cancel the old one?