There's a weapon special ability in the Underdark book that allows you to change the material composition of your weapon at will, once per round. This allowed you to change a normal Longsword into a Cold Iron longsword, Adamantine Longsword, or Silver longsword, or whatever you want. It's a +2 bonus, I believe.
Then there was an ability that allowed you to change the size of your weapon at will also... so a normal dagger could become a large dagger, or a small dagger, or whatever. +1, I think.
Then there's an ability that actually lets you change the weapon itself into any other weapon of the same material (so a longsword could become a Scimitar, or a Quarterstaff could become a Club, or whatever). +2 at least.
So if a weapon had all of these abilities (as it specifically states in the book), that weapon could change from any one weapon to any other weapon, regardless of size and composition, at will, once per round.
