It's not 'change and improvement' - it's 'change for improvement!' The improvement cycle (PDSA) is tried and tested over many years. There are many effective change models. But we often work with them separately, instead of seamlessly, and implementation of improvement doesn't stick. The 'Four Ds' (+1) Model incorporates basic change smarts with the improvement cycle so both are happening at the same time - speeding up the process of making improvement business as usual.