No, you can use your Travel Package cat 6 for any category 6 hotel at the moment you make your reservation till it expires. If you do not use it (or want to upgrade to cat 7) you have to return it for whatever partial value Marriott gives you which is very small since they count the miles at the full value and discount the hotel portion.

Your second question has nothing to do with the changes; this is a known issue for any program and any award reservation. There is no clear answer on that; sometimes the hotel will honor award reservations (or the rewards program will make it happen anyway), other times they will try to put you in a different hotel. The times I've read this and people really wanted to stay at the hotel usually they are able to work something out with the GM and the rewards program. However, if your hotel is suing Marriott.. then I am guessing the relationship is somewhat frosty and it may be a little bit harder. If you dont mind sharing, what hotel are you talking about?