Could a spell effect for an eternal spell be to alter the upgraded state of the target card (change an upgraded card to an non-upgraded card or a non-upgraded card to an upgraded card)? I think this would be useful, but due to the nature of the card, it would not be advantageous to reuse it constantly, as there would be no point in using it on a card more than once (assuming all status effects and damage taken are retained; if not, the card might be used twice to make the card the same as it was when it was played). I think it would be versatile in that you might want to use it on either side of the playing field, to make your opponents card worse or to make your own card better.
Unfortunately, I don't know how practical this would be to make, nor do I know how much the card would cost or what element it would fit into, as I am a relatively inexperienced elements player. Thanks for reading!