a lot of pros and cones here! I made my lively hood in the HVAC industry, and I have seen a lot of changes from when I started a long time ago. I have been retired for sometime now but still help my son out now and then. a lot of things mentioned is true. I remember the old castiron heat exchangers. and match lite pilots. way back before solidstate eloctronics, when things were simple, like our old flatties. the were frendly and easy to work on.
like someones signiture here on the forum! if it ant broke don't fix it.
but my sugustion would be if you won't to do a up grade there are some other
things to consider that wern't mentioned. make sure if you are considering A/C go with the highest SEER rating 1600 or even 1800 with a matcking furnace AFU of 95% or higher to qulify for tax credt. that way the AC will have the new R410 Refrigerant. Lennox, Carrier,Trane, Goodman. these are all good units in my openion. and as mentioned the system is only as good as the instullation. and last but not least. have you contractor do a heating and cooling load on your house, its a must. hope this helps.