OK, just got off the phone with Onkyo customer service about this. I must say, that I got a rep right on the phone (zero waiting), he was polite, knew his stuff, and (by voice) was a native New Jerseyan. So it was a good customer service experience.
Onkyo does have a trade in program, and they will accept a no-longer working unit in trade-in with no issue. The rep told me that they decide what the equivalent current production unit is, and will swap for that.
In this particular case, for a cost of $500, they will ship me a refurbished TXNR818 (not 809), with a 1 year manufacturer warranty, in exchange for my less than perfectly functional TXSR803.
So far as I can tell "street value" of a new NR818 is about $800. Open box, and/or manufacturer refurbished ones are about $700 on Ebay. Used (and functional) SR803 units don't come up for sale that often, but they typically go for about $200. So the upgrade would be effectively, a little better than a fair market value trade. . .assuming I wanted to step up to an NR818 (which, bluntly, I really don't think I do. . .803 is already plenty).
On repair, the authorized service center for Onkyo told me they'd charge me $75 to evaluate the broken unit, then contact me with final estimate for repair. The $75 evaluation fee would go to repair. Turnaround is 10-14 days before they could look at the unit, and then depending on what's wrong with it, and availability of repair parts, it could take another few weeks to repair. More than that, they couldn't/wouldn't say.