Those talks of Yuri replacing Hinata as the 3rd guard student are not bad. It'd be funny to see Hinata snap as her seat is taken.
That's unlikely to happen, at least without a valid reason. As far as guard assignments go, the girls are given their duty based on overall performance in class. The academy has a ranking system and while no explanation was given, we should assume that academics, athletical ability, mental strength and other skills are evaluated. Martial Arts is part of the education, as well as special techniques used by the girls to suppress and regulate their sex drive. Guards need to excel in all of that, because they must be respected and, to some degree, feared by other girls. All so they can enforce order and do their job.
Hinata should have proven her ability so far. So removing her for real would require something to happen. Even a male student leaving school did not result in Miku getting excluded from guard duty. We know Miku was Yamada's guard, they did not really get along and she was likely one of the reasons he dropped out. Yet the teachers let her guard the next guy.
If we look at the class A and E guards, four girls rank among the grade's overall Top 10, and the other two are still Top 20. It should be safe to assume that the other classes are similar. This would imply, student guards are (chosen from) the upper 10% of the girls in their grade and that's where you need to be if you want a baton on your hip. As for Yuri - well, we don't know her class rank. As cute as she is, to me Yuri does not look like someone who had any aspirations to get there. Why would she even, I mean... to guard a guy? Yuck!
Jokes aside - from how I see it, having Hinata replaced would be no easy task. Some scenarios:
• Hinata herself steps down (hahaha, yeah... not happening)
• Hinata misbehaves as a guard or poses a threat to Sou (he'd consider "Horny Hinata" a treat, so that's rather unlikely)
• Sou's family gets her removed - they know he would be upset if they try to, meaning it's a no-go for them
• A girl performs better and takes Hinata's rank (In this case, Miku and Kanaria might veto it, since a guard team needs trust)
• Miku and Kana (with help of Aya's team) rat Hinata out (Would they? Better the sow you know than a female pig you don't!)
• Sou himself asking for it... it's unclear if he can request that. What's clear is, that he has no intention to do it
• can he have 4 guards? I don't think so. They could rearrange the seating, but then he needs five girls - the fight will be ugly.
One other thing that comes to mind about guard teams... what would happen if one of the girls suddenly comes down with a fever or something? I mean, if she cannot go to school then Sou would be left with two guards. Is that allowed? I mean, Sumire would eagerly join, but...