I think they're keeping the GWR ones, but there are no internal changes due on the 379s.Few 12 car runs in the peaks these days too, and I can't see that changing when the more comfy but less numerous 379s arrive. Certainly feels like several 387s are still needed to avoid a net fleet reduction which would go down very badly. Perhaps the 6x 387/3s if keeping just one subclass makes any difference.
12 car operation seems to have gone out of fashion and the time savings, as well as issues when hooking up (such as DOO issues that mean manual dispatch or station skipping), might mean there's no desire to go back.
I suppose if the Sevenoaks services ever ran to WGC all day (and it's purely speculative on my part) then that would help with crowds at the southern end, and give people a choice between train type if they so wished.