Take it to an RFD, it comes off your certificate and goes into their register as a section 2 shotgun. Then the shotgun can be converted to FAC spec.
In the meantime you must get the gap on your FAC for a section one shotgun in the calibre you require. You can then go to the RFD who will then enter the shotgun on to your FAC. His register should state that it was converted back to section 1.
As far as i know there is only a proof requirement for mag restriction.
Also you will have to give good reason for it, i.e. pigeon shooting, vermin control etc.
You will then have conditions applied to that firearm as you would a rifle.