You don't have to change the trim; you just need to change the bulb to one with a tighter beam spread. There is an MR16 with a 10-degree beam spread (narrow spot) and one with a 15-degree beam spread (spot). My guess is that you were supplied with one with a 30-degree beam spread. My recommendation is that you consider an LED version. Then you could have the equivalent of 50W of dimmable illumination for 11.5W of power consumption.