Big-bang or incremental?
Starting from scratch is usually impractical depending on the scale and complexity of the organization. One way to focus resources and minimize risk could be addressing lower risk areas with the potential for largest benefits... the lowest hanging fruit approach.
If an organization is facing failure, risk analysis will change as not implementing the changes carry more risk than implementing a potentially risky, unproven solution.
A continuous improvement effort of a profitable, functioning organization will likely be more conservative and incremental.
Improving organizations could be likened to healing an ailing or unfit body. Medical treatment can range from drastic, to preventive, to cosmetic. In a patient that is not terminally ill, a slow but steady process will eventually tip the balance from malaise to general sense of fitness and well-being and drastic measures driven by desire for instant gratifications are not a wise choice.
The organizational culture, much like the patient's personality, will also be a determining factor in the treatment approach as well as the chance of its success. Professionals tasked with healing will be well advised to tailor the approach to the patient's mindset.