Removing the branding after June would remove the paintwork most likely - so will need a repaint. There are already two back ups available to depots for the TENs. Looking across the Gemini networks I'd probably say Red Arrows needs another spare!
However, 3964 isn't the best bus in the world. This is the bus that has had many breakdowns and engine rebuilds. When it came out newly Northern Red it did a couple of X66 runs and then engine packed in. Came back few days later on a trip to Middlesbrough, packed in. I've only ever got one decent photo of it as a Northern branded bus, as majority of the time is VOR! So having it as a Northern spare for the easy town networks, like the past few days, is probably the better option to it being over the pits all the time!
RE: Go North East