If you are sure the old pass(es) don't have enough days to cover your entire vacation you might as well buy new passes (in advance if you want) to cover it all and perhaps more days and non-expiration. Bring the old passes to check their balances and then set them aside for a time you can get by on on leftovers only.
Example: Your vacation will have six days in the theme parks. Two leftover days will shave only twelve dollars off of your ticket budget when you buy a new four day pass instead. Meanwhile two leftover days will save you $127. (more if you need park hopping) on a later weekend only trip.
Of course if you think you will never go to WDW again you may as well use up all the old tickets as quickly as you can.
Disney hints:
http://members.aol.com/ajaynejr/dispass.htm