The proposal is not about swapping an occupied seat for another occupied seat, that would be too complicated.

I would like a feature where I have the ability to change seats for another vacant seat while inside the vehicle instead of having to leave the vehicle to change seats. that is time consuming and when under enemy fire leaves you very vulnerable.

I would also like a feature where passenger AIs will automatically occupy a turret in a vehicle when its empty.
For example, I was driving a humvee, my AI gunner got shot out. despite having several passengers, I have to leave the humvee and get back inside it so that the turret can get occupied by one of my passenger AI. Or another example where Im driving an APC, the turret is controlled by another player. The player left the turret and I have to jump out the APC so that the turret gunner can get replaced by my AI passenger.

This also is a design choice and won't be changed. We don't want a soldier to be able to operate a vehicle on his own. This is not very realistic and would also affect the balancing.