Pellets, seed and chop are the three things I'd give a green cheek

, with an emphasis on chop and pellets. The chop should contain any number of veggies (there's lots of recipes online and here on ParrotForums.com), minus avocado, onions and garlic as they're poisonous. It can be mixed with pellets. Attitudes towards pellets differ from person to person. I'm of the opinion if the bird will eat it, go for it! I personally mix Tropican with Zupreem cockatiel pellets for my birds. Seed should be fed in moderation. Dehulling is a healthy, mental activity for most parrots (if I remember correctly).
As for converting them to the new food... offer a little of the new food with their regular food and gradually increase the new food so that they're eventually eating the new food as a main diet. Birds in general are stoooooooopid about what they eat. Place a bowl of pellets in a cage all of a sudden and they'll think you're offering rocks. They would rather starve to death. I've Not had that problem with green cheeks. Most of my guys would sample the new stuff the first time they saw it.
Hope that helps ya out a bit

Congrats on the new family member!