Short hop from HKG to NRT in October on CX520.
Booked a month ago, and it was a 333 and selected 39A.

Checking MMB today, the seat is now gone. MMB still shows a 333, but if you try to do another dummy booking - it's now a 772....which of course, does not have 39A. Row 40 is all blocked off.

While NRT isn't as prone to aircraft changes as some of the routes like SIN, it's still highly possible. Especially with quite a few flights a day and this isn't the HND route.

With a flight so far away, would you even bother calling into MPO to change to another alternative seat? Maybe row 54 now that row 40 is all occupied/blocked off? Or just wait till closer to the departure date and leave "39A" on my booking?