I want to change from Priscilla's Diet to the HPW diet. If I just suddenly change their food they can get sick right? Is there a proper way to switch diets?

My vet suggested that I changed it very slow so they don't get sick. Like everynight give them a little less priscilla's and a little more hpw. I am not really sure if it would make a difference if you put them into 2 seperate bowls or all in the same bowl. When I switched diets it was from a real bad dry diet so I didn't really have to worry about how to mix it.

Thanks! Hopefully someone who's switched other diets will see the post so I'll know whether to mix them or not. =] Thanks for your help though!