Najd-Horde-Arabia question

I played Najd for the 500 sunni provinces achievement but decided to reform into a horde mid way, thing is I forgot that I need to do the Sworn Fealty achievement for Arabia as well but I lost my Tribal allegience when I became a horde (I think). Is there a way to revert back to having that government now that I am a horde Arabia? I have enough government reform points. Is it even possible to get it again?

The achievement desc just say Is Arabia lmao I guess I did it