Note whats said above, but thats not my experience I replaced a 2 blade prop with a three blade, approved by FAA under an STC went into CAA who approved it as a minor mod.
Paperwork only took a couple of weeks CAA issued a performance correction for my poh. No fuss.
Replaced obsolete starter with light weight high speed version + copper cable kit + all cly engine monitor added all under STC's all treated as minor mods by CAA.
Have fun!