To enroll a child in a new class, you would go to their profile > Enrollments. From here, click Graduate/Withdraw on their old enrollment and enter in their End Date and Reason. You can also add the End Enrollment Detail if you wish and click Done.
Then you will need to click Add Enrollment and enter in their new class, schedule, etc. Click Save when you are done.
If you need to enroll multiple children multiple children in a new class at one time, you can do so from the Families List screen.
On the Families List screen, use the Filter to select all of the children that will be enrolled in the new class/schedule. The schedule must be identical for all children, so you may need to repeat this step if children have different schedules.
Once the proper children are selected, click on the '...' and select Change Class. Enter their new class, schedule and click Save when you are done.