Can i change my tyre size from 195 65 r15 too 195 50 r15 ?

i want to purchase new black alloys for my car but cant seem to get them in the 195 65 r15 which is the size of tyre which was on the car when i brought it .. but i can find loads sized 195 50 r15 i have been told i can then i cant change sizes does anyone know if i can or not ???