upgrading an transmuted karka shell?
You can better wait until you have an infused version and THEN transmute. As far as I know it you will lose the skin otherwise.
So; Ascended backpiece → transmute Fervid censer on it → Infuse → Lose Fervid Censer skin.
Ascende backpiece → Infuse → transmute Fervid censer ont it → Have Fervid censer ascended (infused) backpiece.
I chose the latter, to be safe and have the skin which I enjoy.
Good luck