A/C changes vents while driving.

Sounds like you have a problem with either the control module for the heat/ac or a leak or failing vacuum motor that controls the blend doors. Have a qualified shop check it out.

It sounds like you have a vacuum leak in the vacuum motors and check valves that control where the air goes in your A/C system. As you go up a hill, there is less engine vacuum, so the doors and motors will change position. Need a savvy A/C tech to locate the leak.