The safest workflow is simple: keep the original, identify whether rotation metadata or frame pixels are wrong, and apply only the changes you actually need.
Work from the original
Do not repair a video that has already passed through several messaging apps or converters if the camera original is available. Each service may reduce bitrate, resolution or audio quality.
Keep the aspect ratio
Changing a wide video to a standard resolution should scale it inside the target frame without stretching faces or interface elements. Padding may be added when the source shape does not exactly match 16:9. Cropping is another option, but it permanently removes part of the picture.
Match frame rate when possible
Converting 30 FPS footage to 60 FPS usually duplicates or synthesizes frames; it does not reveal motion that was never recorded. Keep source FPS unless a delivery platform or editing workflow requires a different value.
Verify before deleting
Play the beginning, middle and end of the result. Check orientation, audio sync, duration and seek behavior. Only remove the original after the new file is backed up and verified.