>People keep trying to say that Spike should never get a song because of Equestria Games adlibbing
>Not wanting him to do a really meaningful "Man or Muppet" style song about himself
>Episode not ending with a Crusaders of the Last Mark status quo breaking like this
I already think Spike's endgame should be defender of Equestria.
So I think the best way to initiate the transformation is to have Spike realize he is capable of fulfilling his dreams. He may not have a cutie mark, but he knows what he wants to be, he just doesn't believe in himself to achieve it.