In addition to selectively opening and closing the vents, you could also add a thermostat so that you have one on each floor.
You could run a switch so that one OR the other is in the circuit. I've been meaning to do this in our house, but I'm just too busy now that I'm retired
.
All the bedrooms are upstairs. The biggest problem for us is when we need the A/C on for humidity, but when it is not all that hot outside. The A/C stays off for a long time, and the heat rises upstairs. I think a thermostat upstairs would help. Sometimes I've left the fan on ON, and that helps, but I'm really running the fan more than I need.
-ERD50