The thing to keep in mind when you have a new holster is that it is your holster now. That means you can do anything you want to make it work best for you.
The holster in the above photo is a Crossbreed Supertuck that did not allow me to grip the pistol. The leather backing is shaped so that you cannot put your three fingers around it easily. I cut some of the leather off but more needs to be taken off. The silver paint marking you see is the outline of the trigger guard. I'm going to cut it there and all the way to the top of the backer.
I have had several holsters I have had to cut on to make them work at all for me. Being a left-handed shooter, I have encountered more than one holster which is really a right-handed holster modified to hang on the left hip. That usually means the magazine release cutout is not really meant to have a lefty button sitting there.
I have also done this because I like to put extended magazine releases on competition guns and those are too long. In the photo below, this holster for a Glock 34 was depressing the magazine release when the gun was in the holster.
How do you know you need to change or cut your holster? The only way to know is to wear it, draw it, and walk around with it for hours and hours if possible. A concealment holster will let you know pretty quickly if there is something you can do to make it better. Discomfort is the most likely thing to experience to let you know. If the discomfort is coming from a place that is not necessary, cut it off.
The video below will show you a number of examples of changes you may want to make. But as you watch it, keep in mind that these modifications are for me and me only. Everyone has a different shaped body and hand so these changes aren't necessarily things you should do if you have the same holster, they are just the types of things you may want to do.
https://www.youtube.com/watch?v=5LRVHs-ii50