We have tickets from Gatwick to SEA in June. I accepted the code change. Nothing else appears to have changed, e.g., we still have our same seats, same class.
Maybe Norwegian is changing the flight "ownership" to the UK subsidiary for taxes, Brexit, and/or something else. A web search shows they have done this for other flights in the past. The warning about maybe losing seat assignments may be generic, again it does not appear to apply in this case.