You can stop your annual season ticket if you want to. You will need to delete the season ticket product from your OV-chipkaart at the transport company that sold you the product. For GVB, you can do this at Tickets & Info kiosks
(as of 22 July: GVB Service & Tickets). Make sure you do so before a new season ticket month begins, otherwise you will pay for a new month. So, if your season ticket runs through to the 15th, delete it by the 14th at the latest. If you paid in advance, the remaining amount will be returned to you.
If you want to change the central zone or star value, your change will go into effect at the beginning of the following season ticket month. So, if your season ticket runs through to the 15th, you can have the new star value or central zone start from that moment. The change zone or star value will the apply for the remainder of the duration of your annual season ticket. If the new season ticket is cheaper, purchase a new season ticket with right details specified and you will receive the difference in your bank account.