I'm not sure I understand - I can see in season 68 Milwaukee Engineering went 29-1 w/ a first round exit in the NT. At the end of that season, their prestige moved from B+ to A-.
In season 69 though, which looks like your first season at the school (or maybe it was SIM coached and just updated to you since you just took over? History tab has always been kinda hokey) - at any rate, that season they went 15-9 with a 120 RPI and didn't make the postseason at all. That's the reason the school moved from an A- to a B, not solely because you took over.
It is possible for a prestige move due to a new coach, but it isn't always a drop. For example, if a successful D1 coach moved down to a D2 school, they'd actually get a prestige bump as a result.