Hi. $500 is a hell of a price to pay for the chance of an upgrade!

I don't have recent experience with flights to Japan, but this behavior is quite common. For some reason UA holds back on releasing upgraded seats until close to the flights. Most of my routes are to/from Europe and SWUs in the end clear, with a few exceptions (major travel periods, most desirable direct flights...). I venture to say you'll be ok.

Did you look if there were flights around your dates with available NC seats?