I think you'd want to be sitting around doing nothing if you were having the train equivalent of major heart, brain and cosmetic surgery all in one go. *Dabs eyes dramatically and not at all sarcastically at the thought of the poor little 170s being rebuilt carefully* Southern would see absolutely no benefit to the new platform extensions if they hadn't had the units now as they need so much work before then!
So what have Southern actually done with these units? They were apparently going to change them from 3 car units in to 2s and 4s but rumour has it that that is no longer happening.
Mind you bearing in mind constant cancellations on both the Uckfield Line (which isnt bad when its shut half the day) and the Marshlink due to train failures Southern passengers would rather like these 170's out on Southern lines if they are fit for use! They might be irritated of they get sent to East Anglia!