I am switching my 8 week old puppy from Diamond brand food to Royal Canin, and have a problem! She is absolutely in LOVE with the Royal Canin food

. I am doing the gradual change (by the way how long should I take to switch her completely?) and she will only eat the Royal Canin. I am going to leave the Diamond food in her bowl until she eats it, but is this normal? Any suggestions??
Jessica