Hi, it used to be done by putting a small spacer under the pressure relief spring, (i.e.a washer or two, I seem to recall flattening a good quality spring washer was about right). :)
I can’t remember how to get at the spring though. :(
You also used to be able to get the rotor kit and extended end housing to make them high capacity. Not sure if it would be possible to dismantle a new Chinese copy for a spin on & retro fit to a bolt on type ?.