I’m skeptical. Although unilateral contract modifications might be allowed, I can’t imagine that one that has cost implications would be allowable. So if a contract says guest will pay $X for Y, with a clause that says generally the terms might change, Marriott can simply change the contract such that guest will pay $N for Y, without guest’s subsequent agreement?

Also, even if such a unilateral change were allowed, where was OP informed that the cancellation fee (or a 1-night fee) would be charged for a delayed arrival? That is still what bugs me about this case. There may be an unstated rule that a guest with a multi-day reservation is considered a no show if they don’t show up by noon of the following day, but nowhere is that stated. And in this case, OP wasn’t really charged the cancellation fee but rather a one-night charge. Where is that stated?