Does changing the gap between plates change the capacitor voltage?
A Wimshurst machine works by that process. 

It puts charge on plates which are close together, then moves the plates apart to generate a high voltage. 

When I was at school, in the '70s, a kid made one using PCB material for the disks, and gramophone needles to create the initial charge. The 'work' was done by an electric motor. Based on the length of spark it generated, I think it produced over 200,000V. 

His dad took it work, where they designed telephones, and tested early electronic telephones with it.