Reforging is a service provided by the Goblin Tinkerer to switch an item's Modifier, or add a Modifier to an unmodified item. Reforging will apply a new, random Modifier to an item, replacing any existing Modifier. Crafting a new identical item will have the same effect, though may require crafting materials not in the player's possession; whereas Reforging only requires Coins.
The Goblin Tinkerer charges the item's current buy value, which is five times its sell value. Existing modifiers affect this value, so items with poor existing modifiers are cheaper to reforge, while reforging an already well-modified item will be more expensive.