If you are going to have a special changeable set, the holy item sacrifice would not be the best template, IMNSHO. The set pieces could be inscribed with runes, which vary the bonuses based on how they are arranged, and could be usable as soon as you find them (though with relatively poor stats). Aligning the runes could require some rare item to limit how many times they can be changed. Perhaps certain NPCs early in the game would be able to partially decipher the runes and give you modest bonuses, while specific higher level NPCs could discern more subtle patterns possible, to give different or additional bonuses.
Perhaps the types of bonuses could be selectable (ie geared to various class types), while the magnitude of the bonus might be based on your reputation or level. A paladin class set could have large bonuses to healing skills or attacks against undead, but drop quickly with any unethical act. A thief set could offer sneak or thieving bonuses based on your reputation among thieves (ie how much you have stolen and from whom).
Aligning all pieces of the set to the same class might give an additional enhancement, or maybe some rune patters (class types) could only be achieved using all set pieces at once (you can mix and match thieves' gauntlets and a mage's helmet, but the elf marksman's archer set of piercing retribution is only available as a complete set).
