If students are already in one of your classes, you can add them to another class with a few clicks! Students won't need to enter a join code or do anything on their end.
Go to your "Classes" tab.
Click on the class name of the class your student(s) are currently in to navigate into the Student Management page
Check mark the student(s) you'd like to move
Choose "Add to classes" from the menu at the bottom of the screen
Select which class(es) to move them into
Click Add for the student to be added to the new class(es) roster
If the student should no longer be enrolled in the older class, you can now proceed to to archive them there following the instructions listed here.