See here for a starter FAQ on HPA:
As per the above posters, converting an airsoft gun to HPA would generally be:
AEG - Designed to operate an electric motor that runs a piston to create a small quantity of compressed air into a chamber to launch your BB
Convert an AEG with an HPA engine, which is fed from an air system.
The AEGs magazine only contains BBs, which means you can still switch magazines as required
Gas - Designed to operate with a small quantity of gas released into the chamber, typically pre-filled in a pressurised compartment of the magazine allowing you to prefill your chosen number of magazines and switch as required
Converting these involves replacing the magazine fill valve with an HPA valve nipple then running a feed from an air system while you play.
As GBB magazines contain their BBs and gas, which gets replaced by the air feed then you can either just use one magazine (which would be the norm) or need to detach & re-attach the airsource if you change magazine for more BBs
Of course you could use a sealed HPA valve and pre fill an amount of compressed air enabling you to fill and swap magazines - but it won’t be capable of holding full HPA pressures and would have limited HPA shot capacity as the stored air decreases pressure with each shot
(other typical gases generally go to a liquid state when filled under pressure in the magazine, expanding in their gaseous state as released to the firing chamber
Dedicated HPA guns are designed from the start to operate with compressed air, and can have their nipple feed in a ‘convenient’ location
These may have integral regulators embedded in the gun allowing for the cylinders standard output to be fed into the nipple, or as seen on most airsoft HPA systems in the classifieds need an inline regulator as well ensuring a lower operating pressure is delivered to the gun
