You got me worried and so I called. My AT&T card letter said specifically 'if you prefer to keep your current card, please call...'
The Citi card letter didn't say anything like that. I talked to another CSR in India today and she confirmed by looking at my account notes that I will not be converted and I didn't cancel my card by doing so (I asked several times to make sure). That was followed by a 0% BT offer (alas with an uncapped BT fee:-(
I'm sure the new World MC would be a fine card perk-wise. The reason why I'm adamant about not converting is the no pre-set limit. I had a Chase card converted to a Signature card not too long ago and sure enough my credit file doesn't show the former 20k limit as available credit anymore. If all your CC's get so-called 'upgraded' to no pre-set limit cards, you end up with no available credit at all, which hurts your credit score a lot.