They didn't rename their planet, they invented space flight according to the guy from Square who thought up this whole idea. They flew from Spira to Gaia. Maybe they'd already drained all the mako from Spira and killed it Or more likely they were just explorers.

They were most likely the founders of the "cetra" who landed there (the Shera airship was probably built by them).

And no, FFX isn't a prequel as such - they're just "connected"