I've done this many times without losing the desired, retained nights. but I prefer to use the online chat so the request is in writing and less chance of a mistake being made. Some times it has taken a long time (up to an hour), as the chat agent has to wait for the modifications team to become available. I just multitask while waiting. Also, you'll want to make sure to see the modified reservation and to confirm that the correct number of points has been returned to your account before ending the chat. If the original reservation was made with a 25% or 30% discount, in my experience, the modifications team may not calculate that in determining the number of points to be returned to my account, so I always ask the online chat agent in advance to remind the modifications team that the reservation was made with a discount and that X number of points should be returned to may account.