I think the basic gist is that the pavilion is the Xandarian exhibition to show of their futuristic technology. You are gonna get on the little ships and use the jump FTL network to travel to Xandar. Somehow Groot or rocket or someone makes a mistake and we end up launching backwards and traveling back in time rather than jumping to Xandar
Indeed, the Awesome Mix Tape show in the Summer sets this up nicely:
1. Earth is on the galaxy's radar (thanks to Quill in the Guardians and that whole Thanos misunderstanding).
2. The Xandarians, as the center of the Nova empire (think what Earth is to Star Trek's Federation), send delegates to Earth.
3. The cultural exchange delegates think Epcot is a real-world World Showcase, and attempt to showcase Xandarian music until Quill shows up with typical European/American cultural imperialism and decries their music as bad and teaches them Earth music (namely, just stuff Quill is familiar, and thus, nostalgic over, which makes this the perfect archetype of Disney entertainment).
And so, this sets up Cosmic Rewind:
1. The Xandarians set up in the part of Epcot that is a showcase of STEM their galaxarium, showing the formation of the galaxy and Xandar and Earth.
2. Guardians somehow get involved and through one of their roguish shenanigans, gets caught up in the demonstration sending the Milano (and us) back to the Big Bang (or a simulation of it) in the roller coaster.
And so, I wouldn't be surprised if the Quill-visits-EPCOT-as-a-child plot point is thrown out, or, at the least, is not the instigation for coming to EPCOT but just a throw-away reference.