In other Forza games, especially motorsport, Upgrading your car to a different class will disqualify it from certain championships. It seems in Horizon 2 that cars themselves are part of the championship (R35 GTR belongs in supercar championship).
Will it stay as a supercar as I upgrade? Will opponents match my upgrades or will I race against class As as an S2? Haven't tested this myself as money is a little tight and upgrades can be costly.