First, cancel the enrollment on the current roster. 

1.  Go to Customers > Account Management > Search to find your customer.
2.  Click on their name which takes you to their account.
3.  Click on the Enrollments tab.
4. Click on the pencil icon to edit the Enrollment.
5. Click the Cancel Enrollment button.
6. Leave the amount as is, the Remove from Roster dropdown should be set to Yes and click the Confirm Cancellation button.

Then, place an order onto the new Roster. 

1. Click on the Place Order button.
2. Click the Add to Roster button.
3. Select all the appropriate values from the dropdowns and click the Add button
4. Click the Checkout button.
5. Click the Done button.
6. Select Payment Method=Other, leave the Amount Paid=zero, add any Payment Comments like "Changing teams" if you want, and click the Place Order button.
7. The player is now enrolled on the other team and their Account Balance is zero.