Just as an update--thank you all for your prompt and detailed replies! I got saved from having to convert any Marriott points or use my other air miles by buying 2 roundtrip economy tickets for travel on EgyptAir (I know, not the best) for $867 each. I called their ticket office and they said it's $900 to upgrade to business class per person at the airport, but we'll see if that's actually the case. Hopefully there will still be some seats left to sell at a discount.