Along a front, which type of air is always forced upward?
No matter the type of front the warm air is always over the cold air, due to density. A cold front is the leading edge of a cold air mass and that mass pushes up the warm air ahead of it.
A warm front is the trailing edge of a cold air mass. In this case, if warm air is advancing faster than the cold air is retreating the warm air moves up the cold air like a ramp.