Jennifer, thanks for posting this link, and thanks for ignoring the obnoxious posters who contribute to the lack of civility on these boards.
Posting casting notices can sometimes be a weird thing. Equity requires audition notices even if a show has no intention of hiring people and does not even need to audition anyone. Not saying that's the case with Hamilton, but you could imagine that the producers are eager to have people hot on the heels of the OBC ready to take their place. The original cast will probably start auditioning for their next projects soon too, since a lot of them can be considered hotter properties as being part of the OBC of Hamilton. But you should expect the cast, especially the leads, and LMM, Jackson, Soo and Odom in particular, to stay on through next summer at the earliest.