
Originally Posted by
Zosi
More properly, you can only STORE pieces from one set at one time. The NPC will take any number of 15/25/35 pieces to store, as long as they're from the same set, which you choose with the chat dialog before trading any pieces. If you change which set you've chosen, you have to take back any pieces of your old set you have stored on the NPC.
Upgrades don't really change anything, but you can't change sets, store more pieces or take out stored pieces while he's working on your upgrade, because he just gives you the "Come back later, not done yet" response when you talk to him.