What the Giants use in week one, they think will work against their opponent in week one. They won't hold anything back. They will, however, give varying degrees of workloads to younger players, allowing them to integrate into the game plans on both sides of the ball, incrementally. I think you saw that last season with Bradshaw. His role expanded only when TC and his staff trusted Bradshaw.
As for preparing for an opponent as the season progresses, same rule applies here as does for any accounting, audit or evaluation: The most recent history is the most important history. If you read what the Giants' coaches had to say about playing Green Bay in the NFC Championship Game last season, they mentioned that they were dealing with a Packer team that was dramatically different from the one that had beaten the Giants in Week 2. It is true that the Giants prepared for the Packers' shuffle screen game. If you remember, the rookie defensive tackle Wright (no longer with the Giants) had an embarrassingly bad time of it being suckered by Favre at the Meadowlands.
There is a fluidness to preparation. You never lose identity as to who you are, how you play, and the fact that you want to exert your will on the opponent. But you know darn well that the other guy is a professional who has been given the tapes of your last four regular season games, and he's going to do everything he can do to thwart what you do well. You can't prepare for an NFL opponent like John Wooden prepared for a NCAA opponent. That is, Wooden completely disregarded any preparation at all about the other team, and simply concentrated on his Bruins playing their game at peak efficiency. It doesn't work that way in the NFL. To some degree, you have to adjust your gameplan to try and counter the strengths of your opponent, while simultaneously seeking to exploit their weaknesses, as evident on film.
Additionally I think familiarity, especially within division, will result in rivals playing each other more conservatively, dare I say more predictably because both teams know each other pretty well, especially if the coaching staffs and key players from each team have been together for awhile. In the NFC East this season, we will have a new head coach with the Redskins. They will use the same running plays that were employed under Joe Gibbs last season. But the passing routes will be quite different. Therefore, the "book" will not be as pronounced.
The Giants have struggled in the second half under Coughlin for a variety of reasons. Injuries certainly have been a factor. Eli Manning does not throw a good ball in cold, windy, rainy or snowy weather. I don't know of any quarterback who truly thrived in those conditions with the possible exception of Jim Kelly . But, in Giants history, Simms was a much better foul weather quarterback than Eli. The Giants will seek to run the ball even more than usual in games played under wintry conditions. But the defense will anticipate this, and will run blitz on early downs frequently. What we don't notice is that offenses throughout the NFL struggle in poor weather. Teams like Pittsburgh play pretty well in the slop of their home turf, but the games are hardly of the high efficiency sort. Rather they're mud scrums.
Looking at this year's schedule, the Giants will play at Arizona on 11-23. That should please Eli. There is also a game in Dallas on 12-14, which is not a bad time to play them in Big D. The season concludes in Minnesota on 12-28, which will be, of course, inside.
Seasons unfold. You're never the team at the end of the year that you were at the beginning. There is the inevitability of change, adjustment and personnel manipulations. I don't think they consciously "save" game plans for later in the season. Looking at this year's schedule, they might try something quite different against the Vikings, I would humbly suggest.