CHT, so in your example, this kid leaves a team where they are (by your description) an offensive-minded impact player; and go to a supposedly better team where:
1) they find themselves playing a totally different position; not all that odd; the player's new coach might not know them that well and wants to see how they do in a position the player is not all that familiar playing at (I've seen players moved around before to positions that, at the time, just didn't make sense, only to discover that the player was a "natural" at the new position); or, the player's new coach might just be trying to work them onto the field in some way, because they might not otherwise get much playing time in what is their "normal" position because he/she is now the "low man on the totem pole" and for the most part, an unknown.
2) they make less of an impact; well, if they left an "average" team, where they were a standout player, for a much better team, there is a very good chance that they will now be just an "average" player (when I say "average" here, I mean similar in skill, etc, to his/her new teammates) on that much better team. If that's the case, it only makes sense that, at least in the beginning, they are going to make much less of an impact on that new team.
"Because development to that point had always been on the opposite side of the ball." Are you saying that this child can no longer develop because they are now playing a different position than they're used to? Not sure I understand that logic.