Besides, there already is a way in-game to downgrade cards, you just don't get any
back for it.
1. Make a deck with the cards you want to "unup".
2. Click PvP1.
3. Profit!
what about BL like previously said?
The cards are permanently downgraded. I learned this to my cost when I accidentally put two Micro Aboms in my PvP1 deck, and then I had to grind again to get the 2 upgrades back.
I might actually do this with my
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nymph, or for that matter with my SoDs (I have 5 upped, 1 unupped).