Hello Jean-Marie,
It isn't currently possible to change Hauptwerk's mixer preset from MIDI pistons, I'm afraid although we do have an enhancement request logged for that. I've also logged as an enhancement request that you'd appreciate a simple means to mute/unmute groups of mixer buses via MIDI pistons for switching between speakers and headphones.
You could currently change the mixer preset using the computer mouse/keyboard (e.g. via the 'Audio mixer, routing and voicing/panning settings' large control panel.) However, be aware that the routing of organs' ranks to bus group numbers is not part of a mixer preset, so you would need to make sure that both of your mixer presets had those particular bus group numbers configured to contain appropriately-routed buses.