[Arran]

 If you want to go ahead and put a later three or four speed changer in there I would try to do so in a way that is reversible, and keep the original changer with it in case you want to put it back to original or have to sell it again later. Right now they are not especially collectable, but they are even less so when they are hacked up to fit a 1970s era BSR into them. As I said before you will most likely have to restore a record changer either way.
Regards
Arran