Well this is one of the reasons we mods/officers are working on and thinking about loot distribution systems now. We are keeping an eye out for anything that helps for when the game launches, like mods. Thankfully Carbine is giving lots of support to the modders.
From what I understand, the 2 locked ones can not be changed, and like Kel's example a more perfect piece may drop on the next boss and you just spent your dkp. HOPEFULLY there will be a loot distribution system that takes this into effect.
the exchangeable parts of the weapon/gear sound a lot like gem sockets in WoW to me, only with more of an impact.