Short answer
Changing to a lower-level course may cause problems for your Student visa. One listed exception is changing from AQF level 10 to AQF level 9.
Students usually need to stay enrolled in a course at the same or a higher AQF level than the course linked to their visa. Moving to a lower AQF level may put the visa at risk, except for the listed move from AQF level 10 to AQF level 9.
Changing from an AQF course to a non-AQF course may also put the visa at risk. In some cases, Home Affairs may consider cancelling the visa.
If you want to move to a lower-level course, you may need to apply for another Student visa first. You should not start the lower-level course until the new visa has been approved.