> Is there a way to "regrade" a scorm activity attempt?
No. (unless you just hack the db to make some changes or write your own script to insert stuff into the db)
I'd suspect that this situation might have occurred if the SCORM package overwrote the existing raw_score/completion status when the student re-entered the SCORM attempt again. Often when returning to an existing attempt the SCORM pacakge will overwrite the score. The "force new attempt" setting can help to prevent this.