It is possible to create a new payment plan for an invoicee who is already set up in Ezidebit. However, an invoicee can only have one payment plan active at a time, so if you set up a new payment plan for them, their existing plan will be cancelled and replaced with the new one. In that case you will receive a warning before the Payment Plan Wizard opens.
If you want to replace the payment plan, continue through the Payment Plan Wizard to create the new plan and delete the existing one.
If you do not want to replace the payment plan, simply cancel out of the Payment Plan Wizard. The existing payment plan will remain in place and a new one will not be created.