Your progress is stored against each individual lesson, not against a position in the list. So an author reworking a course around you does not disturb what you have already done, with one exception that is worth reading to the end for.
Reordering changes nothing
Moving a lesson up or down, or moving it into a different module, leaves your ticks exactly where they were. Completed lessons stay completed and your resume position in each video is untouched. The same is true when an author edits a lesson in place: a re-recorded video or a rewritten reading keeps your tick and your position.
Adding a lesson lowers your percentage
Your percentage is completed lessons over the course's current total, worked out fresh every time you open the course. Add a lesson and the total grows, so the number drops even though you have lost nothing. If you were at 100%, you now have one lesson left to do.
A certificate already issued to you is not withdrawn when this happens. It is permanent once issued.
Your completed status is not re-checked at the moment the course changes, either. The course only re-evaluates whether you have finished when you next do something in it: complete a lesson, sit a quiz, submit an assignment.
Removing a lesson removes what you did in it
This is the exception. Because progress hangs off the lesson, deleting the lesson deletes the record of your having done it, for every learner, with nothing archived anywhere.
- A removed lesson takes its completion tick with it. Your percentage then recalculates over the smaller course, so it usually moves up rather than down.
- A removed quiz takes every attempt at it. Scores, passes and the attempts you had spent all go together. This cuts both ways: a failed attempt disappears too. Quiz and final-exam attempts covers how the allowance works.
- A removed assignment takes the submissions, the uploaded files, the grades and the feedback given on it.
None of that is recoverable from your side, and none of it is something you did. If a course you are taking loses work you needed, ask the instructor first. If they cannot restore it, contact support.
If your percentage moved and you did not touch anything
Reload the course page before assuming something is broken. The number is calculated live against the course as it stands now, so a shift almost always means the course changed, not that your record was lost.