Replacement of 2nd zone furnace

windsor

I have a forced air system and need to replace an 18 year old single stage gas furnace for one of the two zones in my house (2nd floor).  I plan to replace it with a Carrier 3 stage MVC58 furnace with a variable speed fan and the fancy thermostat that is designed to work with it.  The 18-year old single stage furnace for my 1st floor has probably got a couple of years left on it.  My question is does it make sense to replace the 2nd floor furnace with the Carrier unit and not replace the 1st floor unit at the same time.  My 1st floor has two large open areas (family room and foyer) with 16 foot ceilings that effectively sends some heat to the 2nd floor.  There is a 10% discount from the installer if I replace both at the same time