Shevuot 4a Contradictions in Halacha - ומשנה לא זזה ממקומה

Halacha is a living process. Poskim (halachik decisors), like all of us, are works in progress. Their decisions at a given time are snapshots of their evolution at that time. Their later opinions, when different from earlier ones, do not invalidate the reasoning of these earlier opinions which remain a part of our halachik canon.Source Sheet