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.

Mr. Daniels
September 2017