Can anyone explain in brief how difficult it would be to change the 4-lug R56 wheel bolt plate to perhaps a BMW 5-lug wheel? Thanks

 

Either really hard to do it properly (read- change hubs, which in turn requires fabrication, time, money, experimenting, so on) or fairly easy but possibly dangerous (read- wheel adaptors). This has been talked about before and I've always felt that wheel adaptors are a bad idea. Think about it this way, you're trusting a 1/4-1/2 in. wide piece of aluminum to hold your wheels on. Not a good idea.