It would make no logical sense for any ISP to downgrade a connection when a MAC is requested.
It would take a member of staff to make some adjustments to your connection, not a lot of time admittedly but time equates to money. You may then complain so staff have to deal with your complaint, again time equates to money.... and to cap it all you are unlikely to change your mind and stay,...... nor return,......... nor recommend to anyone...and that again equates to a potential loss in revenue.
ISPs are there to make money....why shoot yourself in the foot eh?