I've been watching episodes of JLU this weekend, and I finally got to see the Once and Future Thing, where there's a time shift and John is replaced by Hal for a couple of scenes.
It's a really cool bit, because Hal steps right into the plot, and answers everyone's stares with "Another time shift. I'm Hal Jordan, I'm up to speed. Carry on." John doesn't miss a beat when he comes back. They're so natural about it.
They did this with Kyle and Hal in JLA/Avengers, too. I think I've seen it in a few other JLA stories. There's some sort of DCU law where for the purposes of a time travel story, any Green Lantern can replace any other Green Lantern, and they won't be fazed or confused by this.
I swear I've seen this in almost every time travel story where a Green Lantern appears. We can only hope the entire GLC never gets together during a time travel story, because the readers wouldn't be able to follow the scene with everyone changing positions. I imagine the characters would laugh it off, though. (It's all fun and games until someone get replaced with a Parallaxed version of themselves, after all.)
They try to do this with other characters, but it works best with Lanterns for the variety. I mean, you have 4 Earth-based Lanterns running around as it is. They've all been with the JLA at once point or another, and given the trouble with Earth it's reasonable to assume that any version of the JLA would have dealings with the local Lantern. Even going before Hal you have Abin Sur (and if you screw up the timeline enough, Donna Parker or Carol Ferris would work) and after Kyle there would be any future Lantern idea you could come up with. You can just randomly throw an alien in there from either way.
Anyway, there's not much point to this post. It's just a fun trait of time travel stories.