Hi!
It is not uncommon for the park hours to be released to agents, then the public. The hours are not set in stone. Usually it seems the hours are extended if they are changed. You can normally rely on knowing the dates for Fantasmic and morning and evening magic hours from these early park hour releases. The main changes I have noticed as arrival dates get closer is to extend Fantasmic to two shows instead of one and extend park hours for both DHS and MK. I have never seen them cut hours. I have seen and personally experienced them changing extra magic hours at DHS, but that was last year and have not noticed that happening this season. So, to sum it up, the hours are pretty reliable as they are released with the main potential for change being extension of the current hours offering. Hope this helps!