[Arran]

The only place where I have heard (or read) it recommended to use higher a higher voltage cap is across the primary of the output transformer, I have not heard of it being an issue between plate and ground. In this case, if the diagram is accurate, closing the switch increases the capacity, opening it decreases it since you have two caps in series, it's more a treble cutting control then a tone control. Also since they are in series that means that the total voltage will be double that of a single cap so the only cap that you should be concerned with in terms of voltage rating is the .02 uf one. Tube radio audio isn't generally that bad as long as you replace all of the bias resistors in the audio stages as well as the caps.
Regards
Arran