If you don't follow this advise the upstairs will most likely heat but you will have problems with cooling. The problem you will have is you need more airflow upstairs for cooling and less for heat. Unless you have someway to make sure the needed air is going upstairs(zoning) it won't. If you don't have a good plan you are wasting your money and time and will find when you get done there is hardly any air coming out of the upstairs ducts.
Originally posted by paddyhvac
Its been my experiance that putting a second system or zoning your current system would serve you better in the long run