Can you alter the base item without damaging an enchantment?

I am running a 3.5 campaign and in this game I have decided that I will give out treasure entirely randomly, as opposed to tailoring loot based on the party as I usually do. In this instance, the party seriously lucked out and got a "Holy Avenger" the specific item. There are no paladins in the group, but even the base +2 effect is attractive for our fighter who only has a +1 burning greatsword at level 13. He has died a few times, so he is starting fall behind the rest of the party, who average 15.

We have a dwarven cleric in the party who is a master blacksmith and has “Craft Magic Arms and Armor”, and she asked if she has the ability to change the longsword into a greatsword and keep the enchantment. I don’t know of any rules about this. I want to say no, but I want to be sure that there isn’t some obscure entry somewhere in the encyclopedia of all D&D knowledge offering guidance.

My knee-jerk reaction is to require the cleric to add an additional effect to the weapon; a rule I have already home-brewed which allows items to be upgraded by paying only half the cost of its current effect when adding additional effect. I would call it something like “Morphing”, which would be a +1 effect that just allowed a weapon with this quality to take the form of any other weapon with a command from its wielder as a standard action. But if I do this, then the Holy Avenger, a traditionally +5 effect, would have to become +6, which would necessitate “Craft Epic Arms and Armor”.

Thus I want to ask the internet what it thinks I should do.