Two things.
First, because exchanging these is mandatory, I think it's way better to have it happen right when you play them. Because they're Treasures, you don't really have to worry about exchanging them and then playing the new card on the same turn. I mean it can happen with Venture, etc., but that's rare enough. Or maybe they should be set aside, then exchanged during Clean-up, to make it easier to track how much $ you have, but also easier to remember not to discard them.
Second, each card should be interesting. The concept of "worsening Treasure" by itself isn't cool enough to be worth 10 cards, let alone 30 or so.
