Does this mean I got op-up?
 

On my itinerary I am flying JFK-HKG, HKG-TPE, TPE-HKG, and HKG-JFK.

Now, I had assigned aisle seats on all of them. However, when I checked today I noticed the 3rd leg (TPE-HKG) the seat assignment is gone. Does this mean I got op-up or someone in DM decided that he really wants that seat and I got kick off of it... :)

Most likely an aircraft change. @:-)

From my recollection, I believe you are a Silver member. I am too.

In my experience for Silver members, although op-up can occur frequently, it is never actually processed more than 4 or 5 hours before departure. I think you're talking about a flight that's many days or hours out, and in that case there's no way you've been op-uped this far in advance.

Usually if I OLCI, I will get an Economy class seat number and later on it will change to Business nearer around 4 hours before departure if I'm getting op-up. Or sometimes it happens as late as boarding time.

Also, I've never ever had my assigned seat taken back for any reason except op-up. I think once an SL/GO books, they won't give that seat away to a DM even if they want it.

Aircraft change is also possible though in my experience when they change they aircraft, their IT automatically assigns you a new seat of similar type if possible. Maybe in your case it was not possible to be automatically processed.

I'm sure if you call in they'll be able to answer you.

They change aircraft on HKG TPE every day

I'm quite sure it's an aircraft change, and the fact that it's HKG-TPE or v.v. makes it even more likely. I have this happen to me quite often before, esp e.g. first flight HKG-TPE ~8:30 am, pretty much changing every day or so. and yes, the IT system might automatically assign you a similar seat, but this is not immediate at all - very often it would take a few hours, so I'm actually not sure if it's automatic or manual...

I also think it's an aircraft change.

If you were op-up'd, it'd most likely have already assigned you a seat. For all the occasions where my original seat assignment was no longer shown, it has meant a change of aircraft - sometimes I get a call from CX notifying me of the change and asking what seat I'd prefer.