If you change a campaign after you've already sent it, then the original version is already delivered to inboxes. So that version sitting in people's inboxes is the same as what it was when you sent it, originally. It hasn't changed. Those folks will see the original version.
Furthermore, if they forward the email to a friend, using the Forward to a Friend button, it always sends the version they are seeing at that moment.
But, after you've made changes, if anyone clicks the Web Version button in the footer of your email -- or they visit any campaign URL, with a browser -- they will see the current, changed version.