I am in a very similar situation as I jumped to my new guild, oh about a month ago now. For me, it's the exact opposite, our MT is a DK or my GM which is a pally. But in fairness to him, he does switch out tanking duties.
My advice is this, for me it's never really been about being in the spotlight or whatever. I'm more of a group/guild success type. But, I admit it is hard to keep your ego in check at times. I guess(atleast for me) you always want to feel like you are contributing to the raids success. In the current raiding in Uldar, I don't see strict roles like MT or OT being so concrete as they use to be. That maybe a bigger guild issue for you. In my case, I realize I'll be picking up adds alot, throwing up sunders and demo shouts. I also went to a DPS off spec, because of the abundance of tanks or some fights only needing 1 or 2. My GM asked me to for the good of progression, so I did it. It can be frustrating, I'm not sure there is an exact answer on what to do. The grass is not always greener as they say, and I've come to find tanking spots at a premium these days.
If you are really struggling with excepting a new role, you could try talking to him and getting this all out in the open. If you can't play like that, another guild would seem to be the option. I'd try and get the off night, hell maybe talk with the warrior and see if he'd switch with you for a night, or trade off nights.
Good luck to you.