In my opinion, one of the most common mistakes made when introducing change is to use a transition plan. In my experience, they don't work. The problem with transition plans is it is common to rationalize reasons for why the transition plan cannot be followed, such as a recession or a critical project or feature that must be completed first. Typically, transition plans are something that can be put in place later or are somehow optional. Later may never come. There must be a strong desire for change, and change often means taking a risk or being aggressive. Transition plans are rarely risky or aggressive.
An analogy that one of my colleagues came up with is losing weight. In order to lose weight, you can't talk about the need to lose weight and the need to change your lifestyle by eating better and exercising regularly. Successful weight loss is doing it, not talking about it, starting immediately not at some time in the future or by phasing in better food and more exercise.