[turbo6bar]

+1 to ensuring the system is functioning at peak efficiency.

Assuming you are talking about a A/C-furnace combination as opposed to a heat pump, you can upsize the A/C portion, but it depends on the size of the furnace. The furnace blower will generally have 3 speeds. If your A/C is using highest blower speed, you cannot upsize the evaporator and condenser. If your A/C is using a lower speed, upgrade away.
jurgen