LTspice: Vary a resistor's value over time
Unfortunately using a varistor will not work, as a varistor itself has a dependence on the voltage across it. Much simpler is to right click onto an existing resistor, and to enter a formula. E.g.

R=11-100*time


will linearily reduce the resistance from 11Ohm to 1Ohm over the time of 100ms. You can use almost all functions available for the voltage b sources (arbitrary behavioural voltage source), as well as all kinds of measurements e.g. of voltages of other nodes.