Once you have accepted a task, any subsequent updates are automatically considered accepted as well, so you no longer have to manually do this.
PMs will usually contact you before updating an agreed upon task, to make sure you can take on the additional work. If emails for task updates are switched on in your email preferences, you will also receive an email every time a task is updated.
Fixed value tasks will rarely ever be updated, but for tasks with the PO type Open this may happen with some frequency.