It appears you have covered most of the trouble-shooting items. We know it has worked for 10 years with the duct, blower speed, same blower motor and firing rate. Now, we must decide which of these items has changed; check for obstructions in the duct, or too many registers blocked or turned off, we don't have a firing rate because we are blowing cold air.
It may help to do some cleaning on the furnace. Spiders love to build little nests in the pilot tube, the openings of the burners and other places. If it is a standing pilot, is it burning? or is the hot surface ignitor heating up? Do the burners come on at all? If it is a standing pilot, I would check the thermocouple.
We are still open to further information or questions, and please let us know how it works out for you.