Our Agency (Just over 2K) went from a 5906 to a G4 G17 and my unit (50 members) went from a P226 TDA to a G4 G21. I pretty much did the transition from inception. From my experiences and what we saw; Grip angle was easily addressed in a 2 day transition class. Issues with pushing support side and low was at about a 40% rate, about what you normally hear people complaining about. Grip choice should be based off of performance as opposed to "what feels right". A good instructor should be able to diagnose issues that may be fixed with altering grip length of pull, but unfortunately many generic LE instructors may not fully understand these issues.
On the plus side, first shot or non TDA shot accuracy dramatically increased as well as the subsequent first follow up or second shot fired. Overall qualification scores increased or par times were much more easily attained. As a matter of fact since that time, higher standards have been implemented in agency quals. These benefits were dramatically noticeable pretty much immediately.
Manual of arms is IMO much easier to omit the decocker etc, and going to a Glock then to go the other way around.
I use the slide stop as a release but definitely prefer an over sized stop if allowed. I don't like the factory stop like the 34/35.
DON'T do any alterations that are not departmentally approved. I like to do a lot of stuff to my personal Glocks, but my issued weapons are as issued, period.
Hammer control is obviously a non factor in a striker weapon, however we do still put a thumb on the end plate which tells us that the weapon is in battery or is staying in battery while holstering. We have absolutely gone away from the "you can never look at your holster, when holstering" crap.
We should all be learning to "reset the weapon under recoil" as opposed to "pinning the trigger", but it takes a long time for things to travel through the machine. Unfortunately even the topic of "resetting under recoil" or more so the technique is often confused.
I will just state this again as it is important, don't go outside of policy and be aware when doing anything that may be contrary to a manufacturers recommendation.
Finger placement is very shooter dependent, so just have an open mind. I may have someone go real shallow or sink the finger to the first groove. Just depends on the shooter and various factors. The important part is being able to know this yourself, or have someone smart enough to help guide you. That can be the hard part.
I think it may have been mentioned, but the slide cycles much faster than most can ever comprehend to even remove their finger from the trigger. Also resetting during recoil does not mean that you ride the trigger only the the point of reset. You can or should go beyond reset. There is a lot of slop in a Glock trigger which leaves a shooter a lot of wiggle room to find their happy medium. If you listen to a shooter who is running their weapon fast, try to listen for the audible "click" or reset of the trigger. If you can hear it, they are not resetting during recoil. Again this does not mean riding the trigger only to sear engagement or only to the point of reset.
I don't normally "prep" my trigger to the "wall" on a normal first shot pull, unless if it is for an extreme precision shot. However all of my subsequent trigger pulls will have a reset during recoil and my trigger will be at the "wall" when the sights settle. I don't start my subsequent trigger pulls from full trigger extension even when running sub .25 splits. Many shooters are quite the opposite, but that is often (not always) due to the trigger type. As an example I run a slick 1911 straight draw, minimal overall movement trigger, much different than a Glock.