I would think that you could replace the lighting fixtures without having to replace the existing knob and tube wiring based on
80.9(B)
In fact, 394.10 allows installation of knob and tube as an extension to an existing circuit.
I don't believe that existing knob and tube in hollow spaces of walls and ceiling constitutes an "imminent danger to occupants". Buried in insulation though - well, that's a differant story.
