I thought about that first, too.
I could make a "from -> to" exchange too, but it would need serious thought and obviously some exchanges would not be possible. Not that fun really.
Players might (!) take 1-2 KG and then realize they wanted to play differently - in the end, you cannot test them before.
In most games all things can be changed later on for a reasonable price, and I think this is a good thing, not being tied down. Though (!) having achieved something the hard way is rewarding, too.