Just wanted to add my two cents here. Players are already hesitant to uprade nymphs, as most Tournaments and PvP Events are of the unupped variety. As a good majority of the poll respondents say, the Fire Nymph's ability really needs to be 3
.
This is the only Nymph whose ability cost increases when upgraded, and it just makes no sense. As others have pointed out the unupped version is more cost efficient and Fire Nymphs are already at a disadvantage because they lose their synergy with Angels.
I upgraded my Nymphs during the Masters Tourney (Light Dragons have 12hp) but I'd happily pay 1500 each to unupgrade them now.