LaFell (and this applies to Harrison, as well, if he makes the team) will probably run some of the routes that Hernandez ran. That won't make him a TE, though. If he can't line up off tackle and credibly block in the running game, then he isn't a TE. If the opposing D puts a CB on him, and that doesn't create a major advantage in the running game, then he isn't a TE (this is why I don't consider Jimmy Graham a TE either).
Could LaFell be a TE in the same way that someone like Jimmy Graham is? In name only, but in practice just being a big WR who operates out of the slot frequently, doesn't effectively block LBs, and rarely lines up as a 'traditional' TE? Sure - that's pretty much what he was in Carolina. But in my book at least, that doesn't make him a tight end. To be a tight end, you have to be a credible blocker.