1) Rule changes, this is always the main impetus for change as it involves every player affected by the rule change. If they outlaw bump and run past 5 yards, you are going to create a new emphasis on the type of CB coaches will prefer while CB's who could bump and run down the field will become a lessor factor in the game. etc. etc. etc. for every rule change that occurs.
2) HC's and sometimes OC's and DC's will change their player's style of play and the technique needed to play under the new rule change. Or if a HC creates a whole new offense or defense in the NFL, players will have to change their game to fit in. When HC's brought in the 3-4, it completely changed how we viewed LB's, DE, and DT's, when other HC's brought in the Cover 2 defense, again some positions were redefined, CB's, Safeties, LB's DLmen. Players rarely impact changes in the NFL, rules and new ideas by HC's do. In the old days when say Shula invented the zone defense, it was a major break from the past and created a whole different type of athlete needed to play their positions. Almost every defensive player's responsibilities became redefined.
Ditto for when Al Davis invented the bump and run in the old AFL and then brought it to the NFL when they merged. He used bump and run down the whole field until it was latter changed to only 5 yards.
I don't think the young people realize how much change has taken place in the NFL, did you know that there was a time that the rules were far more like rugby rules where our game developed from. Right through the 40's and even in the earlier 50's I believe, RB's could get up after they were tackled and keep running, they had to be completely tackled before the play stopped and totally unable to get up and run again.
NFL records are a complete joke, not only have the rules changed immensely, the playing surfaces and conditions are nowhere near what they played on in the 50's and 60's. We used to distinguish between a RB who could run in good weather and a RB who could play in the mud much like horse racing does today. Quite of few of the older games were played on surfaces containing a foot of mud with no drainage, a player when tackled could slide for 20 to 40 yards and be covered from head to foot with mud and still play.
Fans of each era love their own stars rightfully so, and think the records they set make their era's players better than the previous stars from other eras. They don't know how much the game has changed and they don't care. They just look at the numbers and think they know what they are talking about.