This. As much as I appreciate building science, the first question I would be compelled to ask the OP is if the upstairs system NEVER kept that area comfortable. If the answer is no, it once did keep things comfy, even on hot days, the system needs attention. If the answer is yes, it never has made things comfy, ever, the solution may lie between HVAC system design and the building envelope.
Originally Posted by hvacrmedic
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.