Sorry, not answering your question, just giving an information how difficult it can be.
You'll have to edit the value of the cheat code and re-patch the game with new values, but it can either be easy, or hard.
it might depends how the patch is done, and whether the game only check the character's level to define the stats, or if each stats are persistent.
If the cheat has each stats information one by one (like str, hp, def, etc.) then you can edit them one by one.
if the cheat only has the "character level" maxed out, you'll have to test, because it's either :
- you can "downgrade" the level, and have the stats reflecting the new lower level, then it will be easy. (less probable)
- If you downgrade the level, but the stats are staying maxed out (most probable), it'll be very hard if the cheat only had the "max level" and not each stat defines one by one, because you don't already have that information to modify manually. In that case, you'll have to search&find cheat code position for each stat you want to edit. The issue is they are maxed out and they can't change on your current play-through so it'll be hard, if not impossible, to find the location in memory. (still possible, though, depends on the game itself)
You'll have to find cheat code using a clean, new savegame, and the game might use pointers (different location in memory for each game restart, each map reloads, etc.), so you'll have hard time to make the newly found cheat work with your existing savegame.
Not impossible, it'll just take time and patience.
Another idea would be a savegame editor, instead of live cheat code activation.