Couple problems I see with this card, though not extremely serious...
First, Holy Light and Nightfall are the ONLY reasons you'd want to use this creature's ability. Rewind would revert the element-change. Were it not for the decent quantum-to-damage ratio, this would fill a niche and nothing more. We don't need more niche cards.
Second, in the one combo it seems to be designed for - Holy Light CC - it is devastating. This is a creature, so it's already going to be dealing damage to your opponent, and Light decks already run quanta-high, so 1
is nothing. It's like packing Fire Lances, except everything but Armagios, Titans, Colossal/Massive Dragons, and boosted creatures can be one-shotted without regard to quanta. It's like having an automatic 30 Fire quanta on your Fire Bolt.
I am also not sure this fits Light's theme. Many people have certainly complained about Holy Light's bias toward a certain two elements; it seems arbitrary and unnecessary. Light is more about healing itself and having indestructible permanents than about destroying things. (Of course Crusader can work kind of like Steal, so Light's theme is expanding...)
I would like to see the Paladin become a bit more flexible. It's a good concept but a bit narrow on the one hand, and possibly ridiculously powerful on the other.