There is most definitely a move back to the 9mm by law enforcement in general, not just the FBI. That said, the vast majority of law enforcement agencies always seem to be light years behind the curve, so the .40 is not disappearing overnight, by any means.
The reasons for the switch by LE are that the 9mm costs less (more budget friendly), carries more rounds in a similar sized magazine, has less recoil (officers are mostly inexperienced shooters, if they have any shooting experience at all, and they are smaller, with less strength generally than a couple of generations ago when departments only hired men and had minimum height requirements), less recoil translates into faster follow-up shots, and it is said that improvements in projectiles make the 9mm perform about the same as LE ammo of other calibers in ballistic gelatin tests, thereby justifying the idea that it has about the same stopping power as other LE calibers. In fact, the FBI's experts now say there is simply no such thing as "stopping power" and that all handgun rounds are insufficient stoppers.
The truth is that unqualified employees, of small stature, simply cannot shoot, and it is all the trainers can do, in many cases, to get someone qualified with a lesser round. There are those who vehemently deny this, especially as to the FBI, who insist that all agents can adequately handle their firearms. Fine, I will leave the FBI out of this discussion of ability of the current hires, but at least as to other agencies, I am not buying that, and I consider it a load of bull. Even as to the FBI, I note that while they issued the .40, the 9mm was issued to those who had trouble qualifying with it. If they were all so good, the FBI would not have had to spend millions in tax money for sophisticated dry-firing weapons hooked to computers to allow agents to see their "flinching" graphically, and they would not have had to authorize a second standard weapon for those who simply could not qualify with the .40.