It's not too difficult. If you knew how to do it before (Just playing around with UOFiddler or even the more efficient route of using MULPatcher to quick patch after new patches) then it's as simple as extracting the MULs from the UOPs, patching/modding them and then re-packing the MULs into UOPs.
Note: With stealth, don't keep the left over legacyMULs in your UO Directory as it will confuse stealth map and affect autowalker and map.