Thanks guys!!!
It worked out!!!

There may be some interesting insights. This is what actually happened.

First I called United (Japan) from my work desk. The lady instantly rejected and told me that I have to re-book the whole thing...
After reading this thread, then I called United (USA 1-800 ....). The lady was very nice, she tried her best to change the way out not affecting the confirmed UG on my way back. Unfortunately my credit card had a Japanese billing address, and she wasn't able to make the transaction, and told me to call United (Japan), saying very sorry.. (Oh don't be...)

Then I called United (Japan) with my cell phone. Now, my cell phone is registered as my primary contact number in United.com, so it directly connected to the premium desk. I asked the agent the same thing and though she said that she cannot guarantee to keep the confirmed UG, she said she'll try her best as I am a 2P (well, still a 2P though ..).

And what happened .... It worked out just fine!! I was able to move the departure date, keeping my upgrade on my way back !!

So thank you so much guys. I hope my experience will be useful for others, too.