Nancy, as you have figured out. Old vents from gravity furnaces aren't designed to carry the air from modern furnace/a/c air handlers and are specially designed to carry an even flow to each room and maintain a certain static pressure.
Second floors are historic problem areas for hvac and are very hard and expensive to fix and retrofit. Someone evidently took the cheap fix by not replacing the duct work along with a new furnace. Talk to a number of hvac contractors and get some bids and remember not all of them completely understand duct work and return design and zoning so be prepared for a variation in answers. New vents may involve some construction/destruction work in walls, etc. to get it right.