With the order placed for what appears to be 26 5-carriage Electrostar units, the 455's are presumably going to be freed up for possible transfer onto non-Metro routes. Two destinations that spring to mind, one having already been mentioned, are Horsham and Tonbridge. Both have a common route through Redhill so could run as a combined train to/from London. It would free up a path out of London Bridge (two if they did away with the Reigate service), which could help when London Bridge is being remodelled.
I've always thought the Caterham and Tattenham Corner services could stand to run combined outside of the peaks to free up a few paths to. This is something that seems to have appeared (in the last year or so) at weekends now so wonder if more combined services are something Southern are considering post 2013.
The Horsham routes are being converted to Thameslink route in 2018 so I think unlikely to be changed to 455's and I believe the 12 coach trains will start appearing from 2016 if they ever get ordered. The Caterham/Tattenham corner services as well but not as combined services. So I think in 2018 the 455's will certainly be spare and to start a new rumour available for transfer to SWT along with the 456's.
Also the Desiro Citys will start arriving in 2016 so maybe the replacements will be earlier on routes into London Bridge or Victoria.
There is going to be a major Timetable revamp in 2018 when Thameslink gets going. I suspect a lot of places will loose their direct Victoria Connection - maybe Tattenham/Caterham and intermediate stations on the Horsham lines as they will be fully served by just Thameslink trains (and with the connections at Farringdon the route to Central London and the West End will change for most people)
On the Tonbridge and Reigate branches from observations at Redhill (my main station) there is very little through traffic from Tonbridge but comparatively a lot from Reigate especially at peaks when Reigate portions arrive in the morning at Redhill with very few seats. However the Tonbridge trains tend to empty and then fill up at Redhill suggesting a good local traffic from Redhill along the branch.
Off Peak the 4 trains an hour to London Bridge used to run relatively empty when introduced but they have been very successful and now even off peak there can be standing at certain times especially inwards of Coulsdon South.
I can't see the 4 trains an hour service from Redhill being dropped in the rebuild and as it is only 2 trains an hour in the peak that is unlikely to be lost during the rebuild - more likely an imaginative solution may be found like splitting trains at East Croydon