Theresa,
Welcome to the forum.
The 401 "should" fit in the 201's cabinet. I have a 201 in a cabinet and a 401 in a case bottom.
The hinges on both machines look to be the same width so that should fit together with out a problem.
You'll have to remove the case bottom from the 401, and make sure the foot controller cord adapter if used is removed so it doesn't get damaged by the cabinet.
You'll have to remove the spring loaded metal plate at the right side of the 201 base in the cabinet as the 401 is longer than the 201.
As far as I can see with out actually doing the switch that should just about be it.
Joe