I can see the Epcot to CBR section being the last to open. It's the longest of the three, construction seems to be further behind the other sections and guests can still take the boats from DHS to Epcot until it's complete, I know it's a lousy option but it's still an option.
This isn't to suggest that they will open behind schedule but I imagine that they will be opening these in stages and not just the whole thing all at once.