Does anyone know about any cast changes for returning Broadway shows and any cast members of closed shows (EX: Mean Girls, WSS, Frozen) getting new jobs?
Each show's website/social media will reflect casting changes once they are announced. It's likely almost every show will have changes, if only in the ensemble.
As far as actors getting other jobs, check the social media of those you're interested in and/or Google their names?
Not trying to give an obtuse answer, but you have asked a hugely broad question, the answers to which largely lie at your fingertips. If that's not possible, maybe tell us who you'd like to know more of and people with inside info might be more apt to share.