So I am planning one of my few international trips ... I only have enough fares to upgrade one way over the ocean. But I think I may have 10,000 to burn on upgrading the connection after jumping the pond.

What is the lowest upgradeable fare class? (NW code or KL code?) And is it a long shot?

I'm pretty sure it's worth it, seeing as I'll be pooped after the long flight and will be glad to have the few extra inches.