If all your rooms are occupied and you want to swap reservations on Bed View, you can easily do that using Switch beds option under right click.
To switch rooms of Reservation A and Reservation B, we do not have to unallocate them and then allocate back to different rooms. We can use Switch beds which will change reservation rooms with just two clicks.
First, select all reservation nights so they are highlighted with red and click on Switch beds option:
Next, move the reservation over to another booking. I move reservation B over reservation A because I want to have it in that room:
Click on the selection to drop the booking and that's done! The bookings will switch their rooms like this:
See also:
How to move reservations on Bed View
How to allocate reservations to specific rooms or beds
Where to find unallocated bookings