But let's have a sense of proportion here, reducing flap from 30 to 20 above 400' in a C172 is safe, and it worked out nicely.
"Safe" is in proportion too. It is
less safe to retract flaps on final, than it is to leave them as set. Though I agree that change in those conditions should not create an unsafe situation, it is taking the plane towards an unsafe condition, where leaving the flaps along would not. It is poor form to get used to doing things on final approach which destabilize the approach, or tend to undo a landing configuration.
The fact that it might have worked out nicely is not justification for doing it. There are a lot of things which can be done in planes, and will work out nicely if done under certain conditions, which still should not be done.
Piloting is very well established now. Aside from new systems, there is not much new about piloting planes. If there is, there will be a procedure and training for doing it. I have never heard of a procedure for retracting flaps during a continued approach to landing. I have heard of maintained a stabilized approach.