I have old tube and knob that is 14-2. Can I use 12-2 to add more length to move a light?
Firstly, 14-2 is a cable, and is not knob and tube. Secondly, unless the circuit is grounded it should not be extended.
Knob and tube, AKA K&T, get its name the the insulators used to support the individual conductors along a framing member, the knobs, and the tubes that went through drill holes. All the insulation will be a fabric material. Some had a loom placed over it where it entered a box.
Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.