Removing sealed units from double glazed windows?

I have double glazed windows and on some of the units the seals have failed and the units need replacing. The point is these windows have no beads only gaskets, but I feel that the units will not come out simply by removing the gaskets. By inserting a screwdriver the units appear to go deeper into the frame so that they would be bigger than the inside of the frame. They were fitted by Everest in 1985.