So, the other day i saw that if you change covenant your renown will set at 0, and i was with The Wild Hunt and get Exalted before change to The Ascended, and i keep my renown with Night Fae, so i made a world quest with the Night Fae and my Renown still continue like it was before i changed with the +renown i get in that quest.
I hope this means that if you get Exalted with your previous covenant you keep your Renown.