I'm going to argue the other end of this one. Swapping abilities is really only a hardware change, which is insignificant in most cases.
For example: Your character favors a longsword and carries a +1. While adventuring he finds a +2 heavy mace. At this point he has three four options: 1. He can sell the heavy mace for 4000gp and pay 2000gp out of pocket to enhance his longsword to +2. 2. He can decide he'd rather use the heavy mace, even though he's had this longsword for a good while and it fits his character flavor, and he possibly has feats centered around the longsword. The mace also has a smaller crit range. Probably not a great choice. 3. He can pay 3000gp and have their enhancement bonuses swapped using the item presented here. 4. Give it to the cleric.
This third choice seems a little expensive in the example given, but the bigger the difference in power, the greater the benefit. I'm for making this a cheaper item, like 1000gp. I think in the past I homebrewed an artificer infusion that basically did this. Its lost now though.
He'd have to pay 6000. 2000 GP for the first +1. +2 is 8000, to go from +1 to +2 you pay to difference, 6000.