depends on the holster.
The 45 is a bit longer so if your holsters are closed and the 9 just reaches the bottom of the holster it might not fit.. They are just a little bit different. I'd call whoever made the holster.
What i have seen from holster shopping is that most, if not all, make seperate for the 9/40 and 45.. I dont know why they would if one could fit all 3 cal.