UPGRADING ITEMS
Epic: 3 maxed Common Power Kits, 1 maxed Rare Power Kit, 3 Rare items (at least 2 of the same type as the item you're upgrading).
Or
17 maxed Common Power Kits, 1 Common item.
Legendary: 4 maxed Rare Power Kits.
Mythical: Lots of maxed Rare Power Kits, 2 maxed Common Power Kits, then a slightly upgraded Common Kit (I cant remember off the top of my head what level it should be).
UPGRADING POWER KITS
Common: 3 Rare items, 6 Common items.
Rare: 3 maxed Common Power Kits, 1 Rare item, 3 Common items.
Note: don't transform the rarity of your Power Kits, just max the current level, whatever it may be.