I really hate to be a Debbie Downer but I just want to reiterate that I really think this is an absolutely TERRIBLE idea. No offense intended at all, I think it's great to talk about different ideas to improve the game. I just think that if you take the single best prize that exists in the game and suddenly make it much less special, you really REALLY hurt the game a lot. If this change happened, I think I would enjoy Elements MUCH less and that it would have MUCH less long-term play value.
I don't understand how it would undermine the game at all. If anything, it would be a boost to deck possiblities and thus create a more dynamic metagame. Also, I think it's safe to say few, if any, have 12+ nymphs. There are players that have been playing this game for over a year and still don't have 12 nymphs or even 8-10 for that matter. This suggestion looks to make nymphs more useful instead of less. Most people thatwin nymphs don't, or can't, make use of them because the ones they have aren't either in the element they would use them for or don't have an amount to satisfy a deck's requirement. Allowing those that have stuggled to win a few nymphs be able to change them up a bit to make cool decks is nothing but an incentive to others to collect enough to make cool decks themselves.
I don't know about you, but if one averages one nymph a month from the oracle, it would take 12 years to get all the nymphs necessary to make all the decks possible with nymphs included and that's a bit longer than I would expect to play to collect them all. Maybe they're people out there willing to do that, but I am not one. I'm sure the intent was to make getting nymphs really hard, but having to wait 12 years to get them all seems a bit ridiculous. Even at 12 in a year seems just a bit overkill, but at least you know people would be dedicated enough to the game to diligently log in every day in that year's time to get the nymph's.
Long story short, be able to shift nymphs from one element to another for a cost seems completely reasonable.