The changes begin soon, in February. To get the full information you should read the 11/8 Al Lutz update on Miceage, it's still available in the archives on the Miceage front page. Or for more fun read the very long threads about it on Mouseinfo's message board where the resident insider there "Marcie" first validated some of the Lutz info but shot down some of the other stuff. But then yesterday Marcie came back on, ate a big helping of crow, and said that Lutz was actually right all along about the changes coming to DCA in '06 and '07.
But here's the Readers Digest version, in a roughly chronological order;
DCA is about to undergo a massive redressing and retheming of most areas of the Park. The improvements and updates they just did to the Hollywood section in anticipation of Monsters Inc. opening are a small scale test of that stuff, and it all went very well. First the main entrance will be changed, with the Golden Gate bridge removed from the monorail beam and the entire entrance area rethemed with California Craftsman architecture like the Grand Californian Hotel. The whole "stepping into a postcard" at the main entrance thing is going away. Giant redwood trees and a lot more vegetation are being planted in that area too, to help with the Craftsman architecture. That all starts in February, and should be done by the start of summer.
Then, in September, the Sun Plaza area gets redone too. The sun icon and wave fountain is going to be removed and sent to the junk heap. The entire area is going to be redone with more Craftsman architecture and the modern look to the area is going away. An old fashioned train station structure is going to be built where the small hill behind the sun icon is now. They will move the Golden Dreams show to a new, smaller theater in that building. The incoherent and jumbled architecture styles currently in that plaza area will be redone and all rethemed to period-specific Craftsman architecture, and a trolley system will be installed throughout the park. The trolleys will leave from the "station" in this area, and the new entry plaza will be a "transportation hub" for the trolleys going to the rest of the park.
Then, likely in winter '07, the Condor Flats area will be rethemed. The high desert airfield theme will go away, and the redwood forest theme of Grizzly Peak will be extended into this area. More redwoods and forest vegetation will be installed, and the sparse desert landscaping will go away. The hangars will remain, but the area will be rethemed to a forest airstrip rather than a high desert airfield. (For those not from the Far West, the high deserts of California and Oregon are rather sparse places, and I don't think a theme park version was really what the paying public was clamoring for. Apparently WDI now feels the same way.) Soarin' stays, but the area just becomes themed to the forest rather than the desert, with small changes to existing buildings and structures and big changes to area landscaping and props.
Then a new E Ticket ride themed to The Incredibles opens in the area currently used as The Farm. The whole farm theme goes away, and The Incredibles will extend the theme of the adjacent Pacific Wharf area instead. The new ride opens around '08, and it's said to be a "family thrill ride", a robotic arm with seats attached to a roller coaster track is heavily rumored to be the ride system used.
During the '07-'08 period, the Paradise Pier area also gets rethemed and reworked. Instead of the modern day look, they'll retheme the area to the 1920's jazz age. The same types of changes that happened to Condor Flats and The Farm will happen here; buildings will be heavily remade and redressed. Facades will be changed, and area accesories will be replaced with ones themed to the 1920's. The Route 66 area is rumored to disappear completely, and since the Golden Dreams theater will be empty with the show now playing in the theater at the park entrance, that's a big chunk of land. The San Francisco area, one of DCA's weakest, will be extended into Route 66. Since the Golden Gate bridge is being torn down in February, San Fran will be beefed up with a new ride and larger area to represent California's second largest city.
There's more, but that about sums up the rumors of what is coming to DCA in the next 3 or 4 years. It all starts with the demolition of the main entrance beginning in February. Some of the "insiders" on message boards insist this is all being driven by executive ego up in Burbank, as they are tired of people making fun of DCA and they want to prove the Eisner era is over.