You can't change the algorithm used to generate the codes once you have put it in use. If you did, it would either A: invalidate everyone's codes from before you changed it, or B: let the people who already hacked their codes retain said hacked codes.
Personally I would say that high levels of encryption are pretty much unnecessary. As long as you don't have things like "RANK" with a numerical integer between 1 and 10 stored there, chances are people aren't going to go through the trouble, or, if they were, they would just open the map and generate their own code.
Don't make any godlike unlockables, and people will be less motivated to hack it. Aside from that, if someone wants to hack it, they will be able to, so might as well spend more time working on the map than the encryption.