I Have Been Taking 5 Mg Of Crestor For Several

I have been taking 5 mg of Crestor for several years. Recently my prescription drug server has ask that I take a generic. My doctor called in a 30 day supply of Simvastatin at 20 mg. I tried Zocor before and could not tolerate the side effects. Why would I be given 15 mg more than I was taking with no side effects? Can I cut these tablets in 4th's without changing the needed results?