Whoa should only mean stop.
Voice and body language are huge tools for lunging. Your commands should be light and crisp up an upward transition, low and firm for a downward transition.
I walk a small circle with the horse as I lunge. My body encourages them forward and I get more of a work out too! When I want the downward, I slow or stop and lower the whip (if I am using one).
If they don't listen to the voice - for an upward transition - move with them - staying towards the hip to encourage forward. For downward, stop and if you need to put more tension on the line. Hand over hand - make the circle smaller until they do break gait. You have to be quick though to ensure they stay forward.
For a horse that won't 'whoa' - we send them to the wall/fence and use the wall as a partner. Can be comical though if our cow bred horses don't want to stop. They roll back and keep going.