Meh this would be abused by
decks with
mark and Nightfall|Eclipse. Don't think it's exactly feasible for a flying eternity anti FG, but definitely alright for PvP. However, I presume that the ability of changing the cards element type will become increasingly useful in the future as I'm expecting zanz to introduce at least a few cards in that direction. In view of that you may want to make it once-per-turn usage only.