What you're thinking about doing would probably help some, but here's something to think about. When you go in a store, or a restaurant, the supply and return diffusers/grilles are also on the ceiling. So, it can work right if done right. I've been told that returns high or low should not make a huge difference in comfort, but ideally you're right, you'd want the returns low if supplies are high. Are the supplies and returns abnormally close to each other? Can you somehow direct the supply air toward the walls/windows (where it is supposed to be aimed toward anyway). I would try other options before going through the trouble to cut those returns. Could not be your major problem (is there a lack of airflow coming through the supply ducts? may be able to balance the system and direct more air where needed).

Where are your returns on the first floor?

If the conditioned cold air is coming out ceiling diffusers it will fall through the room (cold air is heavier than warm air). As it falls through the room it should push the warmer and lighter air towards the ceiling to be picked up by the returns that are high on the wall.

On the first floor your returns are probably in the floor also and that will cause the cold air to come out of the vents and go low across the floor and back into the return grilles.

In many installations if you have high returns in the wall, you also have low returns in the same wall cavity. They are designed to have the high ones open and the low ones closed for air conditioning, and then you open the low returns and close the high returns for heating. Do you have two vents in you wall, one low and one high, that are in the same stud space? If so that is how they are to be used.