From the left navigation menu, click Classes > Class Admin.

From the top horizontal menu, click Classes.

To the left of the class you want to copy the payment plan too, click the gear icon.

From the top horizontal menu, click Class Payment Plans.

To the left, click the Copy button.

Select the existing "Payment Plan(s)" you wish to copy. You can filter the list to find the Plan you are looking for.

Click Copy Selected.

Click Save and Close.