We have developed a DC photoemission gun at the compact ERL (cERL) at KEK for future energy recovery linac light sources and high repetition rate X-ray FELs. The cERL gun has stably provided beam for ERL commissioning and laser Compton scattering experiments since April 2013. The operational voltage has however been limited to 390 kV due to failures of two segments out of the full ten segmented ceramics. In order to recover 500 kV operation, we installed an additional two segmented ceramics on the existing ceramics during summer shutdown in 2015. We performed high voltage conditioning up to 548 kV and holding test at 500 kV for more than four hours. With the upgraded gun we demonstrated high current operation up to 1 mA at 390 kV as well as high voltage operation at 450 kV at the cERL.