Equalizing upstairs and downstairs temperatures when A/C is on
Cold air always sinks (gosh !   :-)  ), so the  stable condition will pretty much always have the lower floor ending up cooler than the upper floor.   Yes,  closing downstairs vents will help balance things out, and at least avoid overshoot in the downstairs temperature.   There should be dampers in the ductwork close to the heat exchanger (in the attic) and those are designed to adjust flow rates for each path.  If those weren't installed, go ahead and partially close whatever dampers are at the exit points of the downstairs vents.