It will be less hassle now than it was a few months ago when you had to get a special extension on your ticket if you started in zone and would finish out of zone.

Why not get them to give you a quarterly 1-3 ticket and than pick up the annual one when you move? Unless you're trying to buy at this year's prices, of course. Otherwise it's going to be £3 a day.

Alternatively, depending on which bit of Forest Hill you live in and which London Terminal you are going to - buy a zone 1-2 and hop on a bus to a zone 2 station. I live near the Horniman but it is quicker for me to get to Victoria by getting a bus to Denmark Hill/Peckham Rye than get the direct train to Victoria.