Vary Capacitance in LTSpice
I remember publishing in 2004 a column in the PCIM magazine (US) where I described how to emulate variable resistors, capacitors and inductors. The capacitor was done in a very simple way and could also work in ac if I remember well. Below is the simple architecture I used and you can probably duplicate it in LTspice:

And if you compare the waveforms obtained with a SPICE capacitor and the varicap component I created, it delivered identical waveforms: