I like the idea of the card, but 2 attack for the upgraded is perfect. Upping the Attack any more would be absolutely unconscionable, because the deck that first comes to mind is Fractal. Fractal once to get 6+ creatures on the field, then fractal again to get 6+ Ball Lightning.
If this thing had an attack of 5, then even if you only had 4 on the field, that would be 25 damage
per ball lightning. Whereas if you had 6 with the other version, it would only be 17 per ball lightning. Still pretty powerful, but much harder as that would be 18
instead of 16
, you'd need more slots in your hand, and any type of DR shield nullifies most of it and able to be balanced with. The fractal Ball Lightning combo is a slight problem with this card, but if you leave the attack as-is it can be balanced for.