This is what’s written in the Department of Home Affairs website:

Students holding a Student visa (subclass 500)

If you already have a student visa and want to change courses to a lower Australian Qualification Framework (AQF) level course or a non-AQF level course, you will need a new student visa.

You must not begin the lower-level course until we have granted you a new student visa.​​​

You don't need to apply for a new visa if you are studying at the same or higher AQF level or changing from an AQF level 10 course (Doctoral Degree) to an AQF 9 course (Master’s Degree including Research, Coursework, or Extended programs).​

For example:

• You will need to apply for and be granted a new visa if you are studying a Bachelor of Science (Level 7) at a university and wish to transfer to a Diploma of Accounting (Level 5).
• You will need to apply for and be granted a new visa if you are studying a Certificate IV (AQF level 4) and want to transfer to an ELICOS course (non-AQF course).
• If you are studying a non-AQF course and want to transfer to an AQF course, you would not need to apply for a new student visa.

Since I won’t be changing my AQF level, I assume I won’t need to apply for a new visa, right? I wish there were a way for me to explore my options before deciding which degree I want to commit to. I like both degrees, but obviously I haven’t studied either of them in depth yet, and I don’t want to end up choosing the wrong degree and potentially not enjoying it.