If you are really unlucky, the bolt (usually offside front) will have corroded into the 'top hat' spacer which results in having to cut the bolt above the spacer and renew....or live with it as is!
I've not encountered the bolts corroded to the ferrule tubes; they normally rust away before that can happen!
On that white 1500HL I broke recently, all 4 bolts were seized solid in the ferrule tubes, the frame still came off! And would have gone back on again if need be!
I find the best tecnique is to drop one end (rather than one corner) at a time supporting it on a scissor jack or similar.