The seat map can be unpredictable at times. Check via the seat map around 6hrs prior to departure and see what the load looks like.

I would then check in as per normal. If an op-up is going to happen, you should know by then.

Sometimes, it might come down to the gate. But this happened rarely in my experience.

If you want to upgrade using miles, let the staff in the lounge work it out for you. They can talk to the gate about releasing seats and getting you your preference.

Good luck. Let us know how you go.