Does it seem correct to change the remaining duration of activities that have not started? For me, remaining duration implies that an activity has started and to change the remaining duration of an activity that isn’t started just wrong.

I would not change the remaining duration of an activity that hasn't started. I would go into the current schedule and change the original duration. This way, if you change in the current schedule you can measure that variance against the baseline that was taken. I think comparing current schedule to baseline (in P6) is a lot easier done than measure remaining duration changes.