Will Citi allow a PC from the Prestige card to one of the Hilton cards or the Diamond card?
You will have to call to find out. It can vary from cardholder to cardholder. For instance, I wanted my Diamond Preferred to be changed to Hilton Honors or Costco Anywhere card but they refuse; they will only offer me Dividend or the suite of Thankyou cards. But others have successfully had their cards changed to Hilton Honors etc.