I'm sure no expert in this area, but here are a few ideas:
1. I
know that if you encounter a dead NPC and use the PC command console to resurrect him, he will appear with all new standard issue out of the box stuff.
So, I'm
guessing that if you killed him (I would use the console to do it) and resurrect him, he would get all new stuff.
2. Reverse pickpocket some permabound armor (perhaps the only viable option if not on PC).
3. Use the command console to add any armor you want to an NPCs inventory. Then use the command console to have them equip it. I have used this before with good success.