He could have left things as-is - all the standard metrics (exams, evaluations) showed there was no dire need to change anything. But true conceptual internalization wasn't taking place.
Where was the problem? Wasn't he a good teacher? And weren't his students of the highest quality?
Dr Mazur discusses the characteristics of the concepts he wanted his students to internalize, the tests he used to test them, the metrics to measure the results and his multiple attempts to change his teaching style to achieve those results.
The process that in the end produced good outcomes (and has been validated in programs throughout the country) turns out to focus less on lecturing and more on directing peer-instruction. Well worth a watch...
Eric Mazur presents Confessions of a Converted Lecturer
There seems to also be good evidence for peer-assisted instruction for elementary level mathematics as well.