Apple Music downgrade from family plan keeping music
I believe she can make a smart playlist of all the music she has in Apple Music and then export those playlist to files listing the songs. 

When you terminate the subscription, she may or may not have a grace period before the music disappears from her library and playlists (which could end up empty playlists). When she signs back up, she would then either not lose anything if there is a grace period or she would import the playlists back and then re-download local copies if she uses that feature. 


https://support.apple.com/en-us/HT205928


The dual membership option with no presumed down time or need to make any changes is documented above. Once your student membership bills, you can cancel the other plan - perhaps timing the billing cycles to have a day or three overlap only. 

A call or web chat to Apple support might be prudent to have them check if you have a grace period or she can sign up herself now and have a more seamless transition.