Did the change to the Ferengi start in TNG?
Well Dax and Sisko did have some history prior to being assigned to Deep Space Nine, besides Montgomery Scott was often referred to as Scotty, McCoy as Bones surely they are just as unacceptable as Dax being called Old Man?
I remember the Ferengi in TNG as is "The Last Outpost" Did they somehow change in TNG?
No they are not. The Captain can use first names when he/she chooses while talking down the chain of command, but for one of his officers to make it a habit and consistently use the Captains first name is inappropriate.