I have done this a couple of times with no issue via Concierge, though it is possible that award availability existed. I did have one issue that the Concierge fixed for me where I booked 2 rooms far in advance and called to change one to GOH. In the interim the Category had changed and my Concierge rebooked the room at the new rate and rounded up my points balance to give me the extra points for the re-booking. I was a) surprised and b) very pleased!