Killing off characters, or even advancing them, won't magically styop people using the old ones (unless the tournament rues do so, for some wierd reason). After all, you can still use the Eltharion on Griffon model in Warhammer, even though his character has moved on, oor the 1st Incarnations of various Confrontation characters, or non-Epic Warcasters in Warmachine ...
Hell Dorado is different in that respect though. As Geof states
here, you will always be able to use an older profile in friendly games, but in official tournaments only the more recent profile will be accepted. Each tournament season results will have an influence on the evolution of the storyline, and of some profiles, as well as on the future advantage cards (cd
interview on TGN), so it makes no sense to play an old profile in a tourney that's been designed with the evolved version in mind.
EDIT after Rogues' comment:
On the other hand, miniatures will ALWAYS be playable in tournament settings. If there's a new version of a model, you will have to play the new profile indeed, but you will still be free to use the old pewter model (ref.
post by Geof)