Honestly, I believe the main differences between Fed's forehand in 2006 and that of 2017 onwards are the change in wrist action right before contact (which was minimal back in the 2000s, as you can see in that well-known video of Roger practising in Tokyo in 2006, where you could see a very smooth swinging motion in the elbow, without much flick of the wrist). Federer's 2006 forehand was, in my opinion, flatter and more powerful, whereas now he has the tip of his racket pointing at the opponent and the head of the racket looking back behind him, to generate spin at the moment of contact (I'll leave a couple links in case you don't get what I'm saying). Also, the buggy whip is also worth considering as nowadays a big percentage of his FHs contain it, if you've noticed, due to Roger's decreasing speed, timing and footwork as well as need for spin.
https://youtu.be/lDJ_2GpAFbY