I like a downgrading option - but only for nymphes.
I´d never upgrade my beautiful nymphes.
btw, in my opinion , the unupped red nymph is better than the upgrade.
The fire nymph can´t be used on cards with 6 health like the Guardian Angel , some dragons ..., heal-rage combo is worse ...
And for other cards - it´s a nice game element, that you have to to think what card you should upgrade.