If it's Kydex (as opposed to Injection Molded Plastic, You CAN reshape any areas of conflict using a Hair Dryer (heat gun) and an Oven Mitt.
Kydex is very flexible and forgiving when heated.
As mentioned, the trigger guard and ejection port are the areas Kydex holsters use for retention. Heat those areas, insert the pistol and use the Oven Mitt to rub those areas. It may take several heatings to get it just right.
Just as an example, I've successfully enlarged a Kydex holster designed for a PF-9, to accept my Shield in about 10-15 min. You could easily reshape the trigger guard and ejection port areas of a G19 Kydex holster to accept an M&Pc.