Hey there!
I brought my GCC Miles (pictured in my avatar) home just over a year ago on the fifth of December. He was on seeds mostly, and he was chubby. He wouldn't eat just veggies, but I needed to get him off the seeds. I'm sure you know since you're asking this, that the high fat content in a seed diet will eventually lead to a premature death.
I looked around for a pellet that didn't have colors and such junk. Roudybush is what I went with. I tried the California blend, but Miles just picked out the dried fruits and left the dried veggies and pellets. So I switched to the Daily Maintenance, which is just pellets. He likes them just fine. He has lost weight down to a nice healthy size. They are high quality and not terribly expensive from Amazon.
As far as the actual switching, I went pretty much cold turkey with Miles. I had to try twice I think, but the second time he just took to them. You can try cold turkey with your bird, and if it doesn't work, try gradually mixing more of the pellets with less of the seeds over several weeks until your bird is eating all pellets. The duration of a gradual switch always depends on the individual bird. Just keep a close eye on the bird's appetite, and if it doesn't eat well for more than about two days, give him some seed back and keep trying.
Always watch for abnormal actions or habits. They could be indicators your bird isn't eating (with a cold turkey switch).
With a gradual switch he will still have seeds to eat for awhile at least. But if you get to the point of all pellets/no seeds and he quits eating, you need to back up a bit and try again.
Just make sure your bird is always eating well. Make sure he's happy with the pellet you gave him. He may not like Roudybush. You will just have to see.
One more thing. If you start out with the gradual method, and your bird becomes picky and isn't eating the pellets you're adding to his diet at all, then go for cold turkey and see what happens.
Hope this helps! Be sure and let us know when you bring your new feather baby home! Do you know what you want to get?