If you have 2-4-2 in rows 21-25 you are in a 4 class plane with NCW and WT+. These are now regularly flown down to the Caribbean (did a return trip in one only last week) Rows 21-24 are the WT+ seats and are therefore the best bet, save for the possibility (likelihood on thse routes !) of a baby or two in the centre block of row 21.

Seat change on OLCI comes and goes from time to time. I guess that others have access to it for particular purposes and that when they are using it the facility is blocked. It often returns 15 mins or an hour later.