Changing the movement is one of the easiest ways of repairing the watch. In reality the customer is paying for a complete new watch which is fine for the majority of the time but on premium quality watches it may affect any second hand value that the watch may have.
I recently saw Longines watch, for a battery replacement, that had been "Restored". The casing had been re-plated, the dial re-painted and a new movement fitted. I didn't know what to say to the customer as I thought the watch was a fake.
I'll leave it up to your judgement when to change the movement instead of repairing the old one

Changing a movement can be very easy if you are replacing it with the same model
Basic Movement Replacement

When the same movement is not available and a clean and service is not appropriate then you will have to put a different movement in the watch.
Complex Replacement

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