The link above to the main thread on FT about this provides the answers in depth. In short:

Any seat in the cabin (Eurotraveller that is), on the same route, with some ambiguity as to whether you can change to LGW or not (see thread). It doesn't need to be in the same fare bucket, and this opens after midnight local time on the day of travel, on the dot. If you do it before midnight you may have to pay a fare difference fee, but of course the buckets change hour by hour so you may be able to sneak in before.