I think the 2 important questions are:
How many workflows do you have to refactor ?
How many of these use vim35 objects ? (If they do not use vim35 objects then they do not need to be refactored).
I have personally chosen to refactor the workflow I needed on a need basis. I took the opportunity to clean them up / rename them in a more consistent way / do them better with looking at the new VC4 API.
If you have tons of workflows then you may consider install a separate vCO 4.0 server, follow the refactoring doc, test the workflows and then export them in a new package.
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