I don't know much about a lot of stuff, but I'm pretty sure Bobby didn't change his style or phrasing due to competition from other players. No one really played guitar like Bob Weir during their heyday...nowadays there are folks in and around the jamband circuit who DO, but they got it from Bobby.
As far as how that song "morphed", well, they didn't play it much different on their guitars from '74 - '95 (a bit faster here & there and they would change WHERE on the guitar they'd play it depending on their mood night to night, I guess), but the only "morphing" that song went through - that I can think of - was the arrangement. They used to put the solos at the end of the song before putting it b4 the 3rd verse.
Finally, I'm almost 100% sure that it's G > C the whole song, but again...you can get away with (and maybe they did too, sometimes) G > Gsus4. I like C best...but taste IS subjective!!


