My initial stay was 6 nights at a Design Hotel, which I wanted changed to 5.The hotel were happy with me shortening my stay. Their initial answer was "talk to marriott". I spoke to marriott on the phone who said "talk to the hotel". It was at this point I posted this topic.After advice here, I spoke to the hotel again who gave a bit more reasoning behind their "talk to marriott" statement. Apparently they do not have the systems, as a Design Hotel, to refund points and so that's why they pointed me to Marriott. Hotel happy to shorten my stay, and said they are happy to speak to marriott at this point. I reached out to Christina at marriott, who very quickly spoke to the hotel, and sorted out the behind the scenes bits to refund my entire initial 6 night stay, then re-debit my 'new' 5 night stay. So, all sorted in the end! Thanks to Christina at Marriott!! Hopefully this will be useful to someone in the future.