Could definitely be done without overhaul.
Gold cost to upgrade = 15 x Selling Price
Items required to upgrade = Reagent of same quality (introducing a Reagent for each quality of item) and a number of copies of the item inversely proportional to the rarity of the item. Common would be say 10 copies, Uncommon would be 6, Rare maybe 4, Ultra rare maybe 2.
Since the game seems to be about items humming with power, the reagents could have various hum strengths visible in the descriptions.
So could use existing rank system, add one new mechanic and an NPC to facilitate it and there you have it a OAD upgrade system that isn't overly complex but requires investment.
I would definitely say cap the items though. Maybe the smithy just converts a brill into a radiant? Unsure the best way to handle it, but that seems right.
If that's the case it wouldn't be a smithy, but a crazy enchanter who now seeks to equip adventurers for his own crazed agenda whilst accumulating wealth. We could name the crazed agenda wealth hungry enchanter Trump, maybe.
