Since they have never done this LARGE of a re-model before it is very hard to guess what we will see in the future, but this is my opinion.
Haunted Manison Holiday-
Has been going strong since 2001, the first year this was done it was not expected to be a major draw and very little attention or advertising was made about the change, until guests saw it. The ride from what I was told by CM's at the time was running at Summer peak lines. They had never seen the lines like this at HM during the Holidays. This has continued to be a huge draw and honestly I can see no reason why they would change that for next year.
IASWH- The current reports are that IASW will be down for refurb. until November, but the plan is to bring it back ready for the holidays. As this has also become a fresh new draw for this old ride. (Especially in my opinion because it is FAR less annoying then the original and actually kind of fun with out the other song)
As to the decorations, they do change with every year, probably because decorations can't all be reused every year, some may get moved whatever. But honestly in some cases you would only notice if you compare pictures of two years (or go to the parks a lot

). Some decorations do change more noticeablly every year like the Tree at DL. 2006 was flocked, this years was not, they also use different ornaments. But they are always beautiful!!!
What will the decorations show in DCA? That I think depends on what they end up actually starting with, since the reports keep changing I will wait and see where the bulldozers start to make a actual call. But I think that DLR probably won't want guests to focus on the construction, so I doubt they will do away with the "holidays" in the park, more just work with what they have at the time.
It should be very interesting where DCA will be at the Holidays. But I can guarentee the parks will have their holiday spirit!!
PS I hope any of this answers your questions
