I'm going through the same thing right now as I am changing my guinea pig to a healthier option. The best thing to do is introduce them slowly as guinea pig's digestive systems are very sensitive. If you would like to, you could see if they eat the new pellets out of your hand. This tells you how fast to introduce the new food. Start by mixing a bit of the new food in with the old one. In the beginning I usually mix in 1 part new nuggets, 3 parts old nuggets At first they may not eat bits of the new food, but continue to do this until you see they start to eat some of it. Then you can start adding more and more until you are left with all new pellets. My pig is very picky, so it usually takes a week before she even touches the new food.