I used to build saddles and rebuild military saddles- McClellans mostly. The traditional way of forming leather to a shape is to "case" or wet the leather, form it around a form and let it dry. We would soak the leather in water so that the leather was pliable. Once wet the leather it will form to a shape, say a bucking roll on a 1970's western saddle, and when it dries it will be in that shape. It will also still be stiff and will not be soft and will protect and reinforce the saddle tree. Using oil or moisturizers excessively will soften the leather and the leather will eventually lose its' form. But when you are trying to form leather around tight curves or compound curves and you still want the leather to be stiff and solid afterward, casing is how I learned to do it.

I've used casing to mold cavalry holsters to cap and ball revolvers and M1916 repro holsters to my 1911A1. I soak the holster in clear water, oil up the handgun and put it in a heavy plastic bag, put the bagged handgun in the wet holster and let it dry two or three days. A good bag and gun oil is essential. When dry I follow up with saddle soap and maybe a little saddle oil (I never use neetsfoot oil).

With this process the holster molds to the handgun. Then you aren't pushing out the holster and wearing your gun's finish when you holster and un holster the handgun. These are unlined holsters so I don't know if it will affect the lining on a lined holster. I know it sounds scary because we are taught to always try to keep leather dry. Repeated wetting and drying will affect leather, but once or twice to mold leather to a shape will not. Find a video of the tanning process and see how many times a hide is washed, soaked and rinsed. While I have no problems casing and molding, I agree with Smoke above that you should contact the maker and see what they recommend.