What modification did the NFL make to the uprights (goal posts) a number of years ago, to help kickers (as well as coaches trying to decide which end of the field to defend during the coin toss)?

They attached streamers to the top of the goal posts to indicate wind direction.

You are correct Leprechaun!! Ribbons helped show the wind direction and intensity. By the way, those ribbons have exact specifications: 4 inches by 42 inches, attached to the top of each upright.