No not what is the ROI.
No not what is the best way to measure change management.
Rather, what does success look like?
For the client:
Of course this depends on who the client is. If the client is the owner of the change, success would be getting to a version of an end state that would not have been possible without CM. Maybe things happened faster than predicted. Maybe the change hangs on longer (or permanently) than originally expected. Maybe stakeholders feel more comfortable about the next change. (See the pattern here leaders? CM is as much about the next thing, or righting the last thing as it is this current change thing… when done successfully).
If the client is in middle of the organization then the future is set aside. Success directly correlates to elimination of roadblocks and speed to success measures (forget end states, and the next change and building anything- in the middle it is about now and tomorrow).
For the Consultant:
For that senior owner it may be that the consultant helped build competency or capability for that owners organization.
For the middle client it may be that all those difficult people issues were handled, dealt with and then things got better.
One is strategic and one is tactical. If you plan on being successful as a change management consultant you better be ready and able to do both. (Ironically sometimes at the same time).