It depends on your status level and how you book.

Generally, you have until 11pm two calendar days after you book online in order to have your ticket issued. But if you book over the phone, you must have the ticket issued immediately. And if you book online then try to change on the phone, they told me this morning that it requires immediate ticketing too (which is a blatant lie, since that was not the case just a few months ago unless the policy changed since then).

The cost to change depends. Generally $150, if you are platinum either $50 or you have two free changes, or if you are diamond you have unlimited free changes.