There isn't currently any way to convert an existing reservation to a Guest or link it to another existing reservation.
Since you're just talking about payments, what I might suggest is that you make a temporary wait-listed reservation (perhaps of a particular Reservation Type so you can recognize them) and put the payment on it. Then when needed you can create a new Guest reservation for that same person as you mentioned, and use Transfer Payment (or Transfer Deposit would be more appropriate, since the advance payment should really be a Deposit anyway) to move the deposit onto that Guest.
You could also just put the initial deposit on the Customer record and not even make a Reservation for them, but it might be harder to locate them that way (not many reports will list Customers, and it wouldn't have a way of identifying them as pending "placement").
Hope that helps.
Stan Spielbusch
[Forum Administrator]