Originally Posted by Points Scrounger

You will need to cancel your Priority Club account altogether first to use the same credit for your yet-to-be-opened United account. If not, the UA one you set up would reject that card.
Easier would be adding a second card (even a debit card with MC/Visa logo is OK) to the Priority Club account as a "place holder," then cancelling the currently-linked card from that account before setting setting up your new UA one.