Good topic!
I would say that changing to a vegan diet can be done without a transition period. There are all kinds of mock meat products and vegan dairy products so people can recreate their favorite recipies and stay vegan.
I think when people run into problems it is when they change their diet to a more healthy diet and also lower their calories to lose weight. When changing to a cleaner diet there will normally be some detox symptoms, and if you also eat at a caloric deficit those cleansing reactions come in addition and can be too overwhelming. So in my experience changing to a cleaner diet is easy, and one should wait until the body has adapted to it before tweaking it further or before going on a caloric deficit to cleanse further:D