It doesn't need an M.O.T. to change the taxation class, just fill in the changes section of the V5 and send it to DVLA, they should then send you a new V5 with the new taxation class "Historic Vehicle". As regards selling it without an M.O.T. you will get a much better price if it has one, as there will be a sudden glut of real crap coming on the market when M.O.T. rules change, if it has a new/very recent test, it will stand out!