I believe in the past an error is generated if you attempt to save samples, activities, etc., with dates outside the range of the workout's start and end date. Are you saying that if I create a new workout and just copy everything over to it, only adjusting the start and end date of the workout, that this will work okay? What about, for example, an activity with an end date past the end date of the new workout?

Yes, I was using seriesBuilder and I don't explicitly finish the route builder, but the route does not get saved if the phone is locked.