During the holiday season, most TPAC flights tend to be full in Y as a result of winter vacation, but widely open in C due to lack of business travelers. This situation alone would lead to mass OP-UPs.

As a result, if you have any sort of status, OP-UPs are very likely, especially right before Christmas from US to Asia, and then the first few days of January from Asia to US.

I have often flown TPAC during this season, and I was OP-UPed everytime as a 2P and later 1P. We are talking about OP-UP when you check in, not those last-minute OP-UP at the gate.

Also, last year, I flew LAX-NRT-TPE. NRT-TPE in Y was so full that they ended up upgrading most people who had status, even a 3P, who sat next to me.

While one should never expect OP-UP or take it for granted, your chance does improve much more during the holiday season.