They aren't going to change the most important aspects of the game (epic Gym/Elite Four battles) that much! And rest assured that they will not have any Sinnoh Pokemon, at least pre-National Dex.
I'm not so sure that they won't. You might be right, but they did change Giovanni's team, and they changed the levels of the E4 for FRLG. Not to mention, these games are in urgent need of a change in rosters, because the game is pathetically easy. RBY was a decent-ish difficulty (infact, FRLG made the game easier, overall, if we discount the fact that the gym leaders in FRLG had AI), but GSC wasn't. In order for me to rate the game as highly as possible, the game NEEDS to be harder. I think others agree.
Of course, there are other ways to make the game harder though, such as having less trainers, so our Pokemon level up much less, and will be weaker in comparison to the gym teams, or decreasing the levels of wild Pokemon, so, again, our Pokemon will be weaker throughout the game, making it harder.
Game Freak usually make intelligent decisions, and I hope they will again.