Replacing Mica caps with film or ceramic?

[Electronic M]

I was lucky enough to get some NOS micas from a place I used to work at so if is fairly rare that I have to order ceramics to replace micas. But I do tend to go for NP0 or C0G when I have to buy replacements for micas. I've seen Banderson and others use NP0/C0G successfully. Sweep circuit caps will be more tolerant to more drifty ceramic temp coefficients than RF/IF/Video stages.

One thing I know from electrostatic deflection set collectors is ceramic caps with non-C0G/NP0 coefficients that are rated around 6KV and ran at 6KV will drop in capacitance. When you run a ceramic at close to it's rated voltage there's some piezo electric effect on the dielectric that drifts the capacitance. Having the voltage significantly overated as you propose should reduce that issue.