Can I Replace A 225/55 R16 Tires With 225/50 R16 Tires?

Yes, you can. However, be mindful of the following details...

1. Your new tire will be about 1" smaller than your current tire/rim setup. This will make them fill less space in the wheel well.
2. Your speedometer will read about 3.5" too fast. This is because the 50 aspect tire will spin at around 30 more revolutions per mile than the 55 aspect tire. (The final drive ratio will also decrease.)
3. Your ride will be harsher as there is less cushion in the tire due to the decreased diameter - unless, of course, you are already aware of this and are compensating by getting a softer tire.

There should not be a brake safety concern because you are not increasing the reciprocating mass of the tire. However, I do have to point out that your traction control's engage point will probably change and this could very possibly negatively affect the performance of the traction control. This will happen because of the change in resistance across the ABS sensor in the wheel bearing, due to the increased revolutions of the tire. If you don't have traction control, ignore this point.