Change an opening when repeated evidence shows that it no longer serves your needs, not simply because you lost a game. Separate the opening position from the result. You may have reached a comfortable middlegame and then missed a tactic; changing the first move would do little to address that weakness. Review the position where your understanding actually broke down.
Most repertoire updates are small repairs. Add an answer to a sideline, correct a move order, or study a pawn structure you keep mishandling. If a London game goes wrong after an early ...Qb6, investigate how b2 and d4 should be defended. You may need one new branch rather than a different White repertoire.
A larger change makes sense when you consistently dislike the resulting positions, cannot maintain the necessary preparation, or want to develop a new strategic skill. Test an alternative in training games before adopting it for important events. Keep notes about the decisions you find easier or harder, and compare more than just your score across a handful of games.
Preserve older knowledge when you update your selections. An opening you set aside can remain a useful second choice, and familiar structures may reappear through transpositions. Review your repertoire periodically: remove redundant practice, prioritise recurring mistakes, and add depth only where you can use it. A repertoire is a working tool that changes with experience, not a collection that must grow every week.
Try this at the board
Find one loss where the opening was playable. Write the actual reason you lost before deciding what to study.