the way it is described in the other thread, the OLCI upgrade bucket is the available J (or whatever other class) seats, minus what revenue management wants to try and sell last minute. e.g. if there are 4 J seats available 50 hours before departure, RM might decide that 2 of them will be available for OLCI upgrade, and 2 will be reserved for last-minute sale.
At T-8, Plat and Ulti (or Ulti only? But I think also Plat) can call and get an upgrade proposal for any available seat, including the 2 that were not available on OLCI.