I think the tricky thing in balancing this, is that its attack and hp can vary so widely.
On the one hand, if you have few cards in your hand, it will be slightly underpowered compared to vanilla creatures
(3
for a 2|2)
However, if you have a full hand (~ 7 cards, 8 w/ extra card draws). It is VERY powerful relative to its cost.
At 7 cards you get a 8|8 or 9|9 for 3
This doesn't even really require a duo to pull off since aether has mindgate and fractal.
The unupgraded version will be fairly OP with regard to cost per stat point. The upgraded version will probably be just fine though considering a 2 to 1 or high cost efficiency isn't uncommon for upgraded cards.
I think the easiest way to deal with this is to raise both the cost and base stats of the unupgraded version.
On a slightly different note, I like that it provides a soft counter to nightmare since the opponent will risk maxing its stats in doing so.