If I get a prepaid annual subscription can I change it to monthly when the year finishes? Can I change it mid year to monthly?
If you get Creative Cloud Student and Teacher Edition (Prepaid) you can't change your plan to monthly plan in mid of the year.
For that you have to cancel your annual subscription and have to restart the Monthly plan.
But if you want to change it to monthly subscription after completing one year (next year) or when the year finishes, Yes you can do that.
You may also check Adobe - Subscription Terms