I took the advice to just not let them cheat me. I went to Johnstone, showed them the serial number and they brought the part out of the back. No questions asked! Everyone was suitably impressed that the original lasted fourteen years. The guy I talked to said when the waranty runs out to come back and he will gladly sell me a new furnace, any of several makes. He said Goodman makes a decent cheap furnace, but their customer service line is "less than stellar".
The cute part was when I tested. I flat forgot to WASH the exchanger before putting it in. When I lit the furnace the smoke near killed me! And all the smoke alarms had a heart attack. But the weather today was a fluke 62 degrees so I just let it run with the doors and windows open.

Thanks again!