I have a wall switch box with 3 switches, each controlling a different light fixture. The box is powered by one hot that is pig-tailed off and connected to each of the 3 switches. The wiring is all K&T.

I am replacing 2 of the 3 fixtures including wiring 12/2 from the fixtures back to the switch. Also, I am running 12/2 from the service panel (15 amp breaker) to the switch box. This means that for 2 of the 3 fixtures, they will be brand new Romex from service panel all the way through the switches to fixtures. What I'm left with is the third fixture, which I am not touching. Which leads me to my question:

Can I run 12/2 from service panel (15 amp breaker) to a switch and then leave the K&T wiring from the switch to the existing fixture? If so, do I have to be concerned with the old K&T neutral in any way?