I would suspect the primary changes would be the bolster/center pin size and location; traction motor cooling duct arrangement and location; some minor issues with traction motor connections, brake cylinder piping and possibly clearances.

There are actually many variations of the 2-axle flexicoil design when you consider all the export models and various gauges. GE has the same issue - there are many major and subtle differences on their industrial and smaller line, and there are several versions of the FB-2 for smaller designs and other exports. I'm not sure how many of the GE designs can be exchanged - I know some that may appear similar can't be traded, yet others can.