Oh I don't know… I think one difference between Epcot and DCA is that with Epcot at least the basic structure of the park is still there, whereas with DCA there was major reworking and construction of entire lands. The bulk of the DCA budget was spent on Cars Land (I heard it broken down as 700 mil of the 1.2 bil on Cars Land, with 300 mil for Racers, 100 mil each on the two flat rides, and then another 200 mil on everything else and the land's theming and rock work. Some of that may be off though). And then of course the main entry land of the park had to be torn down and rebuilt while remaining operational. And there simply isn't anything of that scale and logistical difficulty necessary for Epcot.
I would expect at most we're looking at a couple of pavilions (Energy and Imagination, maybe Wonders of Life) being taken down for work, none of which I would expect to cost more than 300 million. Throw in the occasional smaller projects like Leave a Legacy's removal and film replacements for Norway or Circle of Life, and I'm not sure it's likely that Disney would spend more than a billion on Epcot. I'd like to see Disney go in and redo Spaceship Earth a fifth time or yank out Nemo from the Seas, but I'm not sure it's realistic. I'd like to see a $2-3 total rethinking/rebuilding of Epcot as much as the next guy, but there comes a point where Disney (and me) could see the money better spent, especially when Hollywood Studios, resort transportation, and parts of Animal Kingdom could use a few billion dollars themselves.