A sturdy sports top can make a woman's chest look smaller. It doesn't mean Tifa'a actually had a redesign in that respect.
The 'Keepers of Fate' business ('Watchers of Fate' according to some) are giving me a bit of pause. I hope it's not just some hot nonsense that's being added for the sake of it, but I'm keeping an open mind with few expectations. I'm guessing they might be related to the Cetra in some way. After all, the original story tells us early on that Aerith sometimes hears voices from the planet, and even from the long-dead Cetra themselves. Not much is done with this info until the party reaches the Temple of the Ancients. I figure the keepers/watchers might be the spirits of Cetra who've kept an eye on the world from inside the Lifestream, and they're reacting to Jenova's nascent reunion because it signals the return of their old nemesis. For me, that would explain Aerith's reaction to the creatures we see in the trailer, and it would be an opportunity for this part of the story to 'do something' with Aerith's supernatural connection to the Lifestream.
Either that, or the Keepers of Fate are actually a mistranslation of the Auditors of Reality, as found in the Discworld series, and they've shown up because Jenova and Sephiroth represent an opportunity to make the Auditors' jobs much easier by riddng the cosmos of sentient life.