atm one has to do this in an expensive perFrame previous vs current check.
having an efficient EH would alllow also more systems as result.

params ["_unit","_weaponPrevious","_weaponNew"];
params ["_unit","_weaponPrevious","_weaponNew","_vehicle","_turret"];//for vehicles

Potentially also useful (not sure only previous or also both):

muzzle: String - muzzle
mode: String - current mode of the weapon
ammo: String - ammo

For MagazineChanged probably same setup useful with only weapon in addition