What if your new neighbor is waiting for you to change your seat?

Here's what I would do: If you're happy with your seat, keep it. If there's another open seat you'd prefer, change to it. At OLCI, take another look: If the cabin is full, you'll have a neighbor regardless. If not, you can change (if necessary) to an empty row.

Keep in mind that not everybody chooses a seat at time of booking. Some wait until they checkin at the airport, so there are no guarantees. In J, you can always decide to reseat yourself after everybody is onboard.