Some holsters, both leather and kydex, will accept the .45 Shield and some won't. It depends on whether the bottom of the holster is formed exactly to the slide and barrel of those made for the 9MM/40 S&W. The trigger guard area of the frame is the same on both Shield platforms. If the bottom of the holster is tightly formed for the slightly shorter Shield, the weapon will not seat deeply enough to allow both to work. If the bottom will allow the slightly longer .45 to fully seat to the trigger guard, you are good to go. Only way I know to tell is to try the .45 in a 9MM/40 S&W holster and see what happens.
I do know that the very slightly wider magazine that fits the .45 Shield will not fit in most magazine holders that are made for the 9MM mags. I have a leather one that will accept the .45 mag, but if I use it for that purpose, the 9MM mag will be looser than I like in the same pouch. My kydex mag pouch for the 9MM will NOT accept the .45 mag!