As far as I'm aware, they are set in stone but I seriously hope it isn't because I'm actually gutted that big thunder and dumbo will be shut on our trip if I had known this at booking, I would have selected a different date. They were the main 2 rides I was looking forward to
They can sometimes be extended (as was the case with the big POTC closure in Jan/Feb this year) - but it's very rare and normally they stick pretty strictly to the schedule posted.
If you're hoping for an early opening of Big Thunder Mountain I would say this is very unlikely. Last weekend a lot of scaffolding was up. They seem to be doing a lot of work there.
I was wondering about Space Mountain. It's scheduled to be closed the 17-21. Our trip is the 17-19. I was hoping there could be a chance of a delay of a day or two. No worries, still SOOO EXCITED to go!