They're basically the same, the key difference is when they were ordered and entered service. ie the B90/B92/B2K stock are all the same design but ordered at different times, and the B07 is the newer design for capacity increases.
The B2K batch only dates from about 2001, and it appears standardisation is removing it from service, not its age. (So there will be 2 classes of trains, not a relatively small older 3rd batch)
It is unclear if this newer B2K batch will go for scrap, or be sold off to somewhere else as secondhand.
There are also some growth options in current order, although now that TfL are skint I do wonder if they will consider keeping these for shuttles rather than incurring extra capital costs buying extra new trains, probably not as they seem to be able to get CIL (community infrastructure levy) funding from builders in docklands area