I would imagine they will restructure the managment over all three tocs in south east they run.
As far as i'm aware fcc will revert back to thameslink, gatwick express will stay the same and southern will be southern. Can't see too much changing it will run as it runs now.
They should be replaced with the new seimens trains. Hopefully Southern will also get rid of the 313s but not holding my breath that this will happen quick.
I doubt it will change that much Southern(Govia)used to run Thameslink before First won the franchise. Apart from all trains now being the same colour the service will remain the same.