I'm surprised that no-one has referred to this yet, though it doesn't
seem to appear anywhere on the Southern (or any other) website(s).

I've seen a poster at a Southern station, which has been there at least
a week, that states that from 2 January 2006, when new fares are
introduced, the London's Zone 6 will be extended to include the
following stations to the south of Croydon and Sutton:

SNIP

I don't understand why they didn't just invent a zone 7.

Does this also mean that the Zone A tube stations will also be moving
into Zone 6?

Probably because TfL have a long term goal of having fewer zones, not
more.

Also the inclusion in Zone 6 seems to be a Southern idea rather than a TfL
initiative, probably just to make its charging "fairer" as it has adopted zonal
charging for all its station within London.