Reversing the diode would be a bad idea as the current will have
nowhere to go once the HV switch opens. What you could do is take an
ordinary AC transformer (say 240V primary 12V secondary), go from AC
to DC using a rectifier bridge and capacitor, then feed your 555-based
HV circuit out of this DC voltage. Then connect the +300V to the GND
of your circuit and you have a -300V where normally the GND would be.
Just be sure not to power anything else out of this secondary winding.
Michel
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