Why not call Citi and ask for the product change to get the info straight from the horse's mouth?
It's a quick call when you use the backdoor number. The hitch is going to be whether they'll allow the co-brand card to become a legacy/core card. I know when I wasn't down with paying them an AF on Premier I did it before the 12 months was up but, that's a legacy card.
Been there, done that.... you're welcome!
It gets a bit complcated with specific scenarios and the rules are always changing especially in this climate. Some are more concrete and others are a feather in the wind.
I tried to change my TYP to DC and they said I'd have to wait for a year. I'll call again in December