Board Thread:General Discussion/@comment-24382631-20140110234616/@comment-31121580-20180629224622

Shinigami Source of power is Spirit Energy, Butterfly Aizen is a Transcendent being meaning his reaitsu is magnitudes higher than yamamoto so yamamoto bankai regardless of heat would not affect aizen, since yamamoto bankai is powered by yamamoto own reaitsu which is highly inferior to Transcendent level characters like ichigo and Yhwach as seen with Yhwach 1 shotting Ichibei after awakening his true power.

Yamamoto Could take on Aizen First form and 2nd form as he is still a shinigami once he breaks into Butterfly form he is no longer a shinigami but a creature in a higher dimension.