Changing existing breaker frame size

[john2016]

I have an existing 3P700A breaker that needs to be changed out to a 3P100A breaker. Is is possible to change the frame size on the switchboard (From a M-Frame to a J Frame?) to accommodate the new 3P100A breaker or what would be the best way to accomplish what I am trying to do? See attached picture for reference. The 3P700A breaker is the left side of the switchboard at the bottom. I'm already going to use the two spare spaces that are shown on the side of the switchboard so I can't use those spare spaces.

 

[infinity]

The switchboard manufacturer should be able to supply the necessary bus stabs and a new cover to accommodate the smaller CB. We swap "buckets" around quite often.