If you want to change the room of a reservation in Clerk it's very easy, just go to the reservation window you wish to edit.
Once you are in the rooms tab, click on the room name colored blue, you will have automatically the list of all the room available on those dates (regardless if they are different types of rooms).
By making this configuration all the information you entered on passengers, fare discounts, payments in advance, services and other details will remain unchanged. Only the room will be updated.
PS. If you change the room because you can not extend the reservation days is good to change the room before the dates.