JSF will automatically set the value of listUsersBean.currentUser before invoking listUsersBean.changeValue. You do not have to write your own code to get the selection value. The selection value will be the current value of listUsersBean.currentUser. Guaranteed.
Often the most important part of the news is what they didn't tell.
Keep the examples as short as possible. If they're too long, no one will take the trouble to read them.
Normally, when you submit a form, every input control on the form is also submitted. When using the f:ajax tag, you can code attributes on that tag that indicate that only selected controls should be submitted, instead of all of them.
Regardless, all submitted control values MUST be valid. If even one of them is not, then an error is returned and neither listeners nor action methods will be invoked.
Often the most important part of the news is what they didn't tell.
With a little knowledge, a cast iron skillet is non-stick and lasts a lifetime.