Hourly compensation

I am employed in PA by a large company located in Chicago. I work as a remote service rep. I am non exempt and have always been payed from the time I leave the house. 8:30 till 5pm. Anytime we are out past 5 it's OT time and a half. My manager says he is thinking about put us on 10hr days to save OT. Can he do that? I still have the letter from a long time ago from the hiring mgr. Stating my starting hourly rate and shift. I think my mgr. is just floating this idea to help his OT goals. Can you direct me to a US labor law or someting like that?
Thanks, Angelo

Call the state labor department as a state law may also be involved. 40 hour weeks are Federal.