Usually they have until 6:00 pm Saturday to make any changes to the schedule. After that, they have to ask you about making any changes. If they do change it without telling you and you don't show up there's really nothing they can do about it.
For me, all scheduel changes must be posted before noon on Saturday.
They are required to inform us of any reduction of posted hours at least two hours before the shift.
They are not required to inform us directly of an hour increase.
In general, they need to inform you of changed somehow to make sure you will work or not work them. I am fairly certain that if you are never informed and they try to give you trouble about working the "wrong" time you could fight it.
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that happened to me today. I came in thinking I was only working a 4 hour shift and it turned out that they added an additional 4.5 hours to it. I just rolled with it since I didn't have plans afterwards.