Does anyone know of a way to change the "next" button to an "end" button at the end of a story?

You can actually change this text by using the following JavaScript. All you would need to do is assign a trigger, execute JavaScript when the timeline ends. Replace "REPLACE-ME" with your own text. I can validate that it works in SL3.

NEXT BUTTON
var button_next = $('.slide-control-button-next .text').text()
$('.slide-control-button-next .text').text("REPLACE-ME")
$('.slide-control-button-next').click(function(){
$('.slide-control-button-next .text').text(button_next)
})

PREVIOUS BUTTON
var button_prev = $('.slide-control-button-previous .text').text()
$('.slide-control-button-previous .text').text("REPLACE-ME")
$('.slide-control-button-previous').click(function(){
$('.slide-control-button-previous .text').text(button_prev)
})