According the link provided by Tankee, the 30mm Bushmaster MKII
could be easily converted to 40mm system, interesting 
This weapon can be converted to fire 40 mm rounds with a change
of barrels and minor changes to the feeder assembly. This
conversion can be performed by the ship's crew.
http://www.navweaps.com/Weapons/WNUS_30mm_BushmasterII.htm
Not sure it could be done for BX, but based on the above , it seem
relatively easy which could be done by crew with the proper
conversion kit .
If SAF need more AD cover( against AH for example) for our forces,
some of the BX MKII could be upgraded to use a 40mm cannon.
