Just to add to SBR249's very good summary.
Another easy place to check is Finnair's Manage Booking feature, using the usual BA PNR and surname option, it puts the ticket number right on the top of the screen, you are looking for a number beginning 125 if BA. Personally I would resist changing bookings anywhere other than in BA.com unless you are really running out of alternatives. Mostly it will be fine, but it's possible to screw up seat allocations if you're not careful.