Thanks so much to everyone for trying to help me out. It’s given me a lot of information and a few ideas.

If DL allowed skyteam partner upgrades on Aeroflot I could use either DL miles. Otherwise I could transfer miles from Marriott to Aeroflot Bonus.

i tried booking through Delta but it didn’t work online. I may try calling.

I will also try the dummy booking on Aeroflot and see what it tells me. I’ll probably have to call Aeroflot anyway. I certainly don’t want to transfer any points from Marriott to Aeroflot if I won’t be able to use them. On the Aeroflot website, I get some pages in English but then sometimes I land on a page that’s only in Russian.

This is a short flight between svo and led. But the price difference between economy flex and business is enormous.

Thanks again to everyone.