What would be the point in stealing a 777? There are not that many of them, really, and their history is usually recorded in extensive detail. I think it's unlikely in the extreme that anybody could successfully pass off the aircraft as their own without anybody noticing any inconsistencies.

Maybe the plane wasn't stolen for its intrinsic worth, but what it was carrying.

Not sure if your question is rhethorical, but… a hijacking is when what you are stealing moves on its own (and you use that ability in the process of getting it). If you ask: When does a hi-jacking becomes a theft? as in: it still was a hijacking, but actually should be understood as a theft? Well, depending on what you mean: either when what is being carried inside is more valuable than the carrier (i.e. the hijacking was just a convenient way to get the thing) or, when, oppositely, the carrier is more valuable that what is being carried (and the hijacking was actually the theft of the vehicle, not its content).

In other words: Yes. I’m not sure what you meant, but the answer is yes.