Those T&Cs are interesting. They clearly and reasonably give Marriott the authority to changes the parameters of the program in any way they like. I don't think it is nearly so clear that they give Marriott the authority to change a certificate for future use in that same way. For instance bullet point two specifically refers to "reward or category". That sounds more like a program structure change than the ability to retroactively "withdraw, limit, modify or cancel" an actually existing instrument.