OP, I had a res for 2 nights at an IC and the 3rd night at another chain which was alot cheaper which I thought was around the corner, turns out its an hours walk. So I checked and the IC was available for pts still for the 3rd night, called IHG and told them I now needed a 3 night stay instead of a2 night stay and if she could please add the 3rd night. She said its all done and emailed me the 3 nights confirmation

Well I was just checking my IHG acct for something else and saw under 'My Activity" that what actually happened was my 2 night stay was cxed and the points refunded and a 3 night stay was then redeemed.

So although no damage done, it does seem that when making a change to ares at least with MR and IHG they will cx the old and make a brand new res. I have no idea if my csr 1st checked , I did and knew it was available Yes I could have made a new res just for the 3rd night but at times when a hotel is near capacity it may mean having to change rooms, so I prefer 1 res