I have heard that Citi gives you a lot of pointless talk if you call in and ask to upgrade your Thank You MC to the Thank You Premire. Some CRs at Citi even pretend the card does not exist. High spenders sually get upgraded for hitting about $15K but I also heard that Citi Thank You Prestige cards are issued as Citi Thank You World Elite Master Cards.
Only Citi knows for sure. It doesn't hurt to call and ask. I looked at the terms and conditions for Premier and the default version is a Visa with a minimum line of $5,000. There is also a Mastercard version with a minimum line of $2,000 which may or may not be easier to get. If the Thank you card you want to convert is at least $2,000 I don't see why they would not convert.