This is why I prefer the paradigm of upgrading weapons, rather than just trading them in. An heirloom weapon with a history is soooo much cooler than just some random artifact harvested from a loot table.

You just provide the party with a magical smithing mechanic (flavor to your preference), and whenever they'd find a weapon, it's a smithy runestone or whatever, and they can use it to upgrade or change the properties of their existing weapon.