I'd be surprised if this hasn't been suggested before, but I'll put it out there anyway.
There are a few named weapons which irritate people, because they are cool, but with crummy base weapon types (at least for them).
Basically, this would be an effect on very specific weapons where the weapon would change into another weapon.
Using the ability would open up a selection screen akin to those such as summon monster or resist energy. There, a selection of weapon types would be available (should be very specific, since the weapon is not meant for everyone, just more open).
1st method:
Timer: 5-10 minutes, would work like the Cannith Boots of Propulsion ability, where instead of uses per day it is only on timer. Primarilly for people who switch between 2 different weapon types (like tanks switching from main tank to dps mode).
2nd method:
Uses per day: 1 use/day. Still able to change the weapon type, but not as much as the timer method. (No on the spot modifications)
3rd method:
1 use: looks like an un-upgraded pale lavender ioun stone, where there is only 1 use, and no recharge per day effect. Strict, since the weapon can't be modified anymore, but still more freedom than having to just live with the weapon type as it is now.
Example:
***U12 spoiler here***
the Divine Avenger from the u12 elite VoD is cool (needs a couple mods, plz don't discuss it here), but it is a longsword. Not all paladins wanna use longswords. So lets give it 3 choices, longswords, bastard swords, and great swords. No, not the almighty khopesh, but at least you get some selection.
Now all paladins can have there holy avenger from pnp and use it too!