Hm. Not sure it'll be of any use, but I did convert my MBNA card some years ago. I had an original Platinum Plus card, and I had them convert it to the LL Bean card when that came out. The first issue was very interesting actually: instead of being green like the standard LL Bean cards, it was gray with the card design done only in grayscale. I thought it was much cooler looking, but when it expired the one I got was green. I think by that time with the "card inflation" they were all platinum or whatever.

Anyway, frivolous stuff aside, I know such conversions have happened in the past. I think they're just trying to push the Worldpoints card on you, but the reality should be that an MBNA card is an MBNA card. I dunno about the Amex, but in the end even that should be an MBNA account. I'd try again, talk to someone else. Try telling them you want to cancel your account, as they probably have a retention department. Those folks may have more leverage to make a conversion happen for you. If you're a valuable enough customer to them, I suspect they'll convert to what you want as opposed to dropping you. Of course, this would force you to assume the risk that maybe they don't care about you as a customer, in which case you'll have to decide if you want to go through with the cancel or take the Worldpoints card.

I did something similar a while back and kinda wish I hadn't. I was cancelling an unnecessary UA Visa from First USA/Bank One/Chase (was coming up on fee due; I had gotten it just for the signup bonus), and they sold me on some kind of points card with no fee rather than actually closing the account. Then they forgot to reverse the fee, then when they finally did reverse the fee they forgot to reverse the finance charge. Argh. Meanwhile, that card is in the drawer. I have too many other rewards cards to start using that one right now.