I have an upcoming flight BOS-EWR-MUC booked on Z class. The layover in EWR is 8 hours, I was wondering if I can do SDC to a later BOS-EWR flight with no fee. I am 1K. IF yes, should I call at T-24 before my BOS flight or T-24 before the one I would like to SDC? Also, if no fare class is available in the later BOS-EWR flight, can I ask to downgrade to Y to avoid paying the change fee?