I would not do this - if someone else books the room that is "available" on the website, then you would be stuck putting them into the other room (that the guy vacated). This will cause unhappiness, and will affect your review (accuracy).
An alternative would be to block the room so that nobody else can book it, and then let him use it.
Simpliest way is to have guest cancel and re-book the other room. In other versions of AirBnB it was possible to change rooms even after reservation started. You'd have to send a special request and select the new room to move them in to. If you don't have that other room booked you can move him into it as a kind gesture but don't expect glowing accolades upon review time.