Prestons will give you good data on units prior to 2005. Worth the cost if you work in the replacement market. I wouldn't worry as much about replacement windows changing the loads as I would the potential to improve performance by fixing the duct system. Correct sizing, correct construction, and sealing. If you are investing 6-15K for a new HVAC unit - another 1-3K for an effective distribution system is very reasonable. It will do more for comfort than a round of replacement windows that cost more.
3.1 Model Numbering Systems Introduction All Lennox product is set up with a systematic coding system for both model numbers and serial numbers. Unit model number (including any dash numbers) and serial number must always be used when referring a problem to Lennox in regard to repair parts, literature, wiring diagrams, etc.