This happened to me earlier in the year. I called up and they were able to help me. It happened to me on a non-ps 757.

I called and said that because the NC and NF would go into the same actual cabin (domestic first on a 2 cabin plane), it didn't make sense that they would have availability in NC and not NF, and was there anything they could do. At first the agent didn't want to confirm the upgrade right there, but I kept pleading my case and he finally put me on hold, asked a supervisor, and got it worked out.

I said something along the lines of:
If you are upgrading to coach you should go into business on a 3 class plane--so you should get the NC1 space.

Since it's clearly a two class plane NC should equal NF b/c they go into the same physical cabin. If you take the upgrade space from NF or another person on the international itinerary takes the NC space, it's still one upgraded seat that won't be sold--so they won't lose revenue by giving it to you over the NC person.