There are two basic types of side folder, 3 if you include the AMD.
The German, Polish and Egyptian style use the same rear trunnion as the fixed stock. Change to a fixed stock with those is simple: pull the screw, pull the stock, install new stock of your choice.
The AK-100 type side folder uses a different rear trunnion. To swap that one out you'd have to grind off rivets and replace the rear trunnion.
BSW