A couple of points for clarification/distinction.
Certainly, a player can loose his spot in the lineup to a teammate at all levels. No issue there. Players can also be "called up" from within the organization as well (Think minors or JV players). However, only in pro ball can a roster spot be filled by an "outsider" during the season -- usually via trade (and even then there may be contractual issues). College and HS teams cannot "trade" players between themselves during the season.
Also, don't forget, these "ringers" play for another team they are perfectly happy with. They will return to that team for the following season. Thus, it isn't building the team. It is clearly an attempt to stack the roster to win those last few games/tournaments at the expense of the players who have put in the work since day 1.
Are you sure the ringers are happy with there current team? They may be tgere for the oportunity to be seen by the new coach. I do not remember that being part of the post. Can we know the motivation of the ringers?