I was in the PHVAC business for 27 years, so I can tell you that you need to be very careful about making changes to your duct system.
HVAC ducts are sized to deliver proper airflow so you get even, efficient heating and cooling. The whole system is sized and designed based on heat loss (winter) and heat gain (summer).
I'd strongly suggest that you get a good contractor to look at your system and evaluate what you can and can't do.
Jack