According to WDWNT, Epcot Cast Members will be the first to experience the new exhibit, Walt Disney Imagineering Presents The Epcot Experience before it opens to all guests.
The previews will take place on the 28th, 29th and 30th September, 2019 from 9:00am to 6:00pm. Opening on the 1st October, 2019, guests will be able to see all the exciting plans for Epcot at the above-mentioned exhibit;
“Inside this first-of-its-kind offering within a Disney park, guests will discover engaging and interactive exhibits that allow them to step inside the relentless innovation, energy and excitement and see some never-before-revealed details driving the future of Epcot all throughout this unprecedented period of transformation.Jessica Figueroa, WDWNT
Everything you need to know about the future of Epcot can be found here.