Sorry, I may be conflating you with someone else. As you pointed out, the old usernames who posted each bit has changed.
I'm really saying that U18 positions are not "real" positions like they are in AFL. For example, taking Carlisle or Hooker forward, or playing Stanton or Jerret back is definitely changing their positions because they've played 5+ years in those positions (at either AFL or VFL level), with the associated training, preseasons, tactics training etc.
However, I don't see an U18 player in the same light. They tend to get moved around, some coaches play for wins others for drafting, some have rubbish around them while some are squeezed to other positions. They're still growing, both in skills and bodies. They don't have the history of playing a single position at a high level with tactics. Unless we're going to start considering U16 form in a whole new light.
So I believe they are far more malleable. Now, there are some players who will have played exclusively a single role, and with their attributes seem best suited to it. But there are many who for one reason or another (often their body isn't quite ready) will be playing roles that isn't their future one. Especially in the championships where they're trying to gel a side from a whole heap of sub-teams.