Quote from Skreemaster:
Perhaps you could add some kind of magic red button that switches the layout between the new and the old...
With an intermediate knowledge of php and phpBB's code, that would take you about one and a half hours to install. Add about twenty minutes for each additional phpBB template you have installed on your forums. This is a lot harder to do than the attachments MOD, and sometimes won't work on phpBB 2.0.11 and up (these boards run off of phpBB 2.0.13).