I’m pretty sure a summoner can’t just change the shape of the atronach they summon. The summoned atronach remains the form it had when it was summoned from Oblivion, be it a Flame Atronach from Infernace or a Storm Atronach from Levinace. It’s just like with other summoned Daedra, like Dremora.

Nevermind, sorcerers can change the shape of the atronach; but they still have a ”default” shape (being the one we commonly see in the games). Daedra can change shapes, but not simply at will when it comes to Lesser Daedra.