If you want to transfer the room which is already used in the reservation, you can not delete it from one room type and add it to another one. Therefore, you need to do the following:
1. Go to Settings / Hotels / Rooms section.
2. Next, select the room type you are interested in. Enter the editing menu of the room you want to transfer by clicking the edit button or the name of the room.
3. In the editing menu go to the Settings block at the bottom of the page and set the Enabled? switch to "No".
4. After that, the room will be unavailable for booking. Now you need to create the same room in the desired category. Select it and click + Add room at the top right corner. If you need to transfer several rooms you can use the room generator.