From May 20th, LO services won't be using the turnback at West Croydon on Mon-Sat. Those services will cross over to Platform 1 as Southern will be using the turnback for their Victoria to West Croydon services via Crystal Palace.
ELL services will continue to use the turnback and Platform 3 and 4 on Sundays
I too am wondering this, I never understood why this wasn't the case from the beginning.
It's quite likely to be the result of the wider mass timetable reshuffle related to the new Thameslink timetable which requires them (and Victoria service) to be passing over Windmill Bridge junction at precise times to avoid screwing up the ECML and MML services.
While that may only seem to relate to fast line services - don't forget the crossing moves made by East Grinstead services (and may be others) that tie the fasts and slows together.
That phrase about butterfly's flapping wings causing earthquakes elsewhere springs to mind.
In other words while on the face of it the switch of platforming may not seem to have anything to do with a Thameslink, if you dig deep enough it probably is.
Its just dawned on me... a major irritation on using LO when going to Croydon is that the journey time between Norwood Junction and West Croydon varies widely, with some trains always stopping for a few minutes so as to allow another train to go in front. Maybe, just maybe, by not using the same platform the LO service will experience less delays when approaching West Croydon.