Uniform is set by TfL and won't change (well, supposedly we are getting the new uniform after the new concession starts, West Anglia already have the luxury!) I haven't seen any actual internal documentation saying that it will change from LOROL, just the ownership of LOROL.
New uniforms specified by TfL already issued to staff on West Anglia Inners. Will also be issued progressively to staff on the original lines from late-summer onwards. Not dependant on new concession as this is a TfL specified requirement.
Apart from the uniforms I hope that there will be no changes to the brand or livery; - 'London Overground' sums up exactly what it is!
The branding and livery are tightly specified by TfL, so there will be no change to that. From a passenger point of view, near enough nothing will change.
I agree we are unlikely to see "thunderbolts and lightning" on the day that Arriva take over but we do know that a shedload of contract options have been pre-specified by TfL and no doubt priced by Arriva. I do expect to see some off peak frequency improvements on bits of West Anglia before long. I also expect we might see some Boxing Day services at Christmas 2017. They might even manage something for Christmas 2016 but that clearly depends on the scale of planned engineering works affecting the Overground. Beyond that we are obviously looking at the resumption of GOBLIN services and then the phasing in of electric services and new stock on West Anglia / Watford DC. That then allows more intensive peak services on the NLL. That's likely in May 2018 (at my guess) but depends on how quickly the Class 710s are brought into squadron service.
We will also have a progressive delivery of enlarged stations / step free access at a few locations but that's a separate initiative to Arriva's takeover.