How are you judging this, out of interest?
I wouldn't expect to hear members of the general public refering to individual Underground lines by name - just "the Tube" or "the train" or "the Underground" - unless giving specific directions or referring to disruption affecting a particular line or part of the network, which is exactly the purpose of the line names. Much in the same way that I would generally refer to getting on a bus, rather than getting on a number 11, unless there was a need to be more specific. I wouldn't expect usage to be any different for the Overground - reference to the mode (train, Overground) most of the time, reference to the line (Windrush, Lioness etc) when needing to be more specific. Obviously some people will take longer than others to regularly use the new names, which is no different to any other situation where people continue refering to pubs, shops and other businesses by older names.