Changing credit card for an existing booking seems to be impossible with Hilton. Two weeks before my recent USA trip visiting 5 different hotels (Hilton, HGI, Doubletree, Hampton Inn) I brought up all my bookings and changed the credit card. The change was accepted. When online check in was available the day before, it was still my OLD card showing. I changed it again. When arriving at the hotel, they still had my old card! That was at all 5 hotels!
Really frustrating!