Abruptly Changing Chinchilla Diet

So I am taking care of my class pet chinchillas over the summer and I’ve done a good deal of research and now know what chinchilla foods are healthiest. However, my teacher who owns the chinchillas (I probably know more about them then him though tbh) says they like the food that has other stuff mixed in with colors and nuts and stuff. For the summer though I’d like to switch them to a healthier diet. Is it safe to abruptly change their diet with no transition period. Will they still eat and be fine? Also should I switch the food at all considering they are likely to go right back to their old food with no transition when I return them to my teacher in late August? Is the food with colored stuff mixed in with the pellets really that unhealthy?