Whatever the announcement is, it is not exclusive to WDW, or else they would be having it there. I suspect that it has something to do with all of the companies theme parks, not just the one in Florida.
Why did WDW celebrate the 50th? It's not the park turning 50.
Disneyland did. But in coming up with a worldwide marketing campaign, Disney could encourage potential park guests to visit the Disney park closest to them, with each park getting something new and special for the 50th.
That's what I think this announcement tomorrow is all about. A new way to celebrate Disney theme parks all over the world and keep guests coming even though the 50th celebration is completed. It's not smart business for Disney to pour all their resources into one destination, but by doing different events at different parks according to the strengths of each destination, Disney doesn't neglect it's other parks. Disney could have kept the 50th focus on Disneyland only, and that would have done just fine, but imagine a WDW without Everest and Soarin'(brought for the 50th). Is it still wonderful and fun? Yes, but the worldwide celebration enabled Disney to allow guests who don't live close to Disneyland the opportunity to join in on the fun.
That's what I think tomorrow's announcement is all about. A new way for Disney to encourage guests, no matter what park is closest to them, to visit Disney. It's not just about 35 MK and 25 EPCOT, although both parks may wish to commemorate those milestones. It's just that those milestones mean very little to someone who's never been there.