Renewal creates a new order, identical to the current one, at today's catalog price. The Renew button shows on the panel three days before expiry.
With no option ticked, renewal delivers new addresses. The old ones stop working when the new period starts.
With "keep the same addresses" ticked, renewal returns the same lines, with the same username and the same password. The choice is stored on the service and can be changed at any time.
After expiry the service stops delivering and the button stays, now with the state Expired. Renewing at that point follows the same rule: new lines, or the same ones if the option is ticked.