I consider Epcot a Must See park, especially when traveling with kids older than 5. Our son has loved going to Epcot since he was 6. While we visited earlier than that, he really became interested at that age.
Actually there have been many changes. While the exterior of the buildings will look the same, many new attractions have been added to Future World. Test Track, Mission Space and
Soarin' are headliner attractions. Depending on individual ride tolerances, these rides are considered Must See attractions. Personally, I skip Mission Space but I wouldn't go to WDW without riding Soarin'. Just hearing the music makes me emotional. Spaceship Earth has recently gone through renovations and some of my fellow panelists have given it rave reviews.
Our son also likes Journey into Imagination and exploring Innoventions. The Seas with Nemo and Friends is a fun pavilion. Your kids will have a “totally awesome” time at Turtle Talk with Crush.
Get autographs from characters and/or World Showcase Cast Members. You can purchase “passports” (or make your own) and have the Cast Members from different countries sign the books.
The finale to your Epcot evening should be watching IllumiNations: Reflections of Earth (which premiered in 1999). This fireworks, laser, water and light show is breath-taking.
Epcot is a very large park. I would break up your day with a rest back at your resort pool. Have a magical vacation!