I'm trying to consolidate pedals. I was wondering if it was possible or not to change the function of the same exp./vol. pedal depending on the setting of a given preset on the G-system?
No it is not possible to have one single pedal controlling volume in some presets and expression in others.
The volume can only be controlled globally in the G-System. If a pedal is assigned to control the volume, it will be a global assignment.
Expression (control of effect parameters) can be different in each preset.