Regardless of moving records or retention of callings, there's also the underlying issue of watching over the YSA's during their innumerable transitions (perhaps they're going home, or doing a summer job like pest control sales). I can't imagine everything a YSA Bishop juggles at the end and beginning of semesters, but ideally I think both the YSA Bishop and the home ward Bishop, would want to be in communication regarding each individual YSA that is coming-and-going. And in a conversation about each individual YSA, the Bishops together can decide what's best for the YSA -- what's recently happened in the past couple months of their life, if they've started/stopped attending church regularly, any hallway discussions or in-person interviews, etc. Granted, that's a lot when you multiply it by the number of YSA's the Bishop is trying to minister to.
Three summer months in the life of a YSA is a mini-lifetime, and a lot can happen. And communication can help the Bishops together determine how to apply the GHB policy that @russellhltn shared, along with the considerations that @mevans describes (like potential worthiness interviews a particular YSA might need, that only the Bishop where the records resides can perform).