DaveP wrote

I am thinking of buying a new Zone 1 - 4 annual, and then trading
in
my Zone 2 - 4 annual. Is this possible, and how much will I get
back

You might find they base any refund on the cost of monthly and weekly

tickets - so as though you had been buying monthly tickets. This
probably
[...]
I came across this when I went into hospital some 8 months into an
annual.

There is in fact a different rule for exchanging a season for a
different one, eg on a change of home address or job. This is based on
the number of days and so gives much better value but it may only apply
to tickets issued under the NR conditions of carriage - I can't see
anything about it in the Tfl CC.

--
Mike D