Screens are either held in by a rubber seal, or bonded with an adhesive...
Mk1's and 2's are rubbered in, and easy to swap over, using a length of cord, anything later, is bonded in, and will require a specialist to carry out the work
Its about an hour or so's work to do either, and cost would be around the £100 mark, give or take a few quid for either
Thom