If a desk agent attempts to change a room in this situation OnQ will throw up a large warning message basically asking you if you really want to do this. Also, Hilton's OnQ training that every desk agent is supposed to go (granted I did it a long-long time ago) through teaches you to never preemptively change a room that has been chosen by a guest unless they ask for it or there is some overriding reason that room is unavailable. Technically you're not locked into anything and any agent can change whatever/whenever but there is no reason/incentive for them to do it.