The student's status on a question is stored in the database as a value between 0 and 1. The student's score (status*weight) is calculated on the fly when it is needed (e.g. in the Student Progress and Scoring Tools screens), so changing the weight does not affect the data recording how much of the question the student got correct.
If you're quite paranoid, you can take a backup of the entire course from the Course Administration page, which would give you something to go back to if things go wrong.
Danny