I answered this question below. But, assuming one is replacing a furnace simply to obtain the advantages of today's high-efficiency systems, there is a less obvious factor to consider.
I'm told by experts that the high-efficiency units often have a shorter average lifespan of about 12 years because the heat exchangers are made of thinner metal (and the heat exchangers cannot be replaced). So homeowners may want to plan ahead.
If, for example, you make the upgrade when your youngest child is entering first grade, you may have to install another furnace at about the time that child leaves for college.
What a coincidence! This also may be when you start thinking of downsizing to a smaller home. As a result, you may be buying one furnace for yourself and a second one for the people you will sell to.
So, form a financial perspective, it might be good to wait until that youngest child is a little further along in elementary school.
Prudential Real Estate of the Rockies