A change in branding can work as a way of disassociating a product from a brand that has a level of toxicity. Marketing firms make a lot of money from refreshing or reinvigorating a product's market appeal.
The vision of the sectorisation branding was, in part, to banish the countrywide joke that British Rail had become in the media.
Refurbished, rebranded trains do fool some passengers into thinking the train is new, I once had a big row with a FB friend over whether the Class 321 Renatus trains were brand new and a similar disagreement when First Great Eastern refurbished the same units back in the late nineties.
Greater Anglia immediately improved perception of rail services after the dark days of "ONE/National Express East Anglia", introducing an app, ensuring trains were cleaner and lots of bright red and white which looked far fresher than dour white and navy blue (possibly why I support Arsenal and not Spurs).
The fleet renewal gave GA the opportunity to apply a new modern livery styling inside and out thus putting clear light between the passenger experience old trains and new. Perhaps the best example of a railway rebrand was Intercity, from the notorious BR sandwich to a quality operator of fast services in less than a decade and still the best livery BR trains have worn in my lifetime. It survived a remarkably long time too with some Mk2s in East Anglia still wearing it in 2005.
NSE initially didn't want its new colours on any unrefurbished trains or non-corridor units (though some did sneak through) opting just for the symbol and brand name with Blue/Grey, which explains why it took 10 years to banish BR Blue and Blue/Grey trains from its network.