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.