There is already two 10 car services from Guildford to Waterloo via Cobham. The 7.16 and 8.07.SWT also published this a while ago, which needs some reading between the lines:
Although it doesn't expressly mention 458/5s it clearly describes how the nett result of the 707s arrival is that Reading will get longer trains, (and that's known to be a 10 car project) and 'mainline' services will be increased and lengthened. So that latter bit needs 450s, and the only realistic place to get them from is the Reading line.
I hope more carriages will also be available for the 18.39 Waterloo to Poole. It's currently only 5 and that's not enough between Waterloo and Woking if they want people to not be standing the eighth of the train. A 450 would be better in terms of space for standing but they are only 4 carriages.
I can't remember if 4 carriages is as good as 5 in terms of numbers of people.