Nothing is ever a guarantee at WDW. Posted hours 6-7 months in advance are always subject to change, and they usually do. My last trip they added hours the week I was there, that week. But of course that was a positive so nobody would even think of complaining, right? They change things all the time, sometimes for the better for some, and sometimes that means its the worst for others. These after hour events do benefit some, but of course there are those who feel it will negatively impact them so they are the most vocal in their complaints about the event. It is impossible for Disney to make everyone happy all the time. If you are happy about a change, chances are someone is not.
You brought up party dates, if you make your plans before party dates are announced and you are planning on going during party season, you should expect that your plans will be effected. If you make your plans after party dates are announced, you should expect that your plans will be effected. Remember, everything is subject to change.
To answer your question, how would I feel, I'd feel disappointed but that is all on me. I plan a trip so far in advance, knowing that everything is subject to change, and it does, well that is how it goes since I wasn't guaranteed anything.