You can move blocks from one building to another using the Transfer blocks command accessed from the Edit menu when the site contains more than 1 building. This starts the Transfer blocks dialog below. Select the building containing the blocks to be copied and the building to receive the blocks and Press OK.
You should check before starting the transfer that none of the source blocks overlap with blocks in the destination building. Even a small overlap will cause the process to fail.
Note: the operation cannot be undone and it clears any existing undo/redo information.