Can I change how a backlog item with tasks is estimated?

Yes, you can change the task estimation options in the backlog item. To do that, open the popup dialog for the backlog item, and click the link "Estimation options" below the tasks of the item:

Note that these options are available only if a backlog item has tasks.

Using the estimation options, you can configure whether the tasks in the backlog item should be estimated or not. You can also configure how the estimate and remaining work of the backlog item itself should be determined.

You can also configure the default task estimation options in a project. You can do that in the popup dialog for the project, in the Projects page:

The default estimation options will be used for all new backlog items that you create in the project, and for all existing backlog items that do not yet have tasks. Once tasks have been added to a backlog item, use the estimation options in that particular backlog item (in the popup dialog for the backlog item) to configure how that backlog item is estimated. In other words, you can safely change the default task estimation options in a project without affecting any of the existing backlog items with tasks.