Plastic wrap and Leather Lighting on the inside of the holster, Leather Lightning will make the inside of the holster slick and the plastic wrap will ever so slightly stretch the leather so that it will release the gun but retain the molded (boned) shape without softening the leather. Never use anything that will soften the leather. I have several horsehide holster that I have done this with and the gun snaps in like it would with a Kydex holster. You may not even need the plastic wrap. Leather Lighting can be found in the link below.
I've only had to stretch one holster, but I wrapped the pistol in a plastic bag and left it in place for a weekend. It's loose enough now that it works just fine.