If your bird is not already on a balanced diet, it may be a little difficult to convert him. Difficult, not impossible! The first thing you will need to do is decide what kind of schedule to start the conversion on. Do NOT take away his seed diet!
The best conversion method I have tried is as follows:
First thing on rising in the morning, feed everything except the seed, nuts, and pellets. These should be kept separate from the other items you feed your bird. Ideally, you have two feeding dishes and one water bowl. Fill his bowl with only a small amount of soft foods and chop them rather small. He will probably throw out most of the items unless you get real lucky right away.
After about an hour (less if the foods are ones that spoil quickly) remove the food, and place a very small amount of the seed, nut and pellet mixture in the cage. If you work in the daytime, just leave this all day. Parrots eat best in the early morning and late evening.
If you are home at midday, try a small amount of the other foods again, after removing the seed dish. Leave them for the same amount of time. This is the hardest part. Don’t feed the bird anything else until suppertime! Of course make sure he has plenty of fresh water. You may however leave a few vegetables, like raw broccoli or carrots, that won’t spoil quickly in his dish.
Then, at suppertime, fill his dish with all the goodies you want him to eat. Some people find that a few seeds mixed in with the mixture will entice them to try it. Remove before spoilage as before.
Lastly, about one hour before bedtime, place his seed mix back into the cage so he will have had plenty to eat for the day.
After a while, your bird will become used to the fact that if they don’t eat the good foods along with the seeds, they will go without for several hours of the day. Persistence and patience is the key. Eventually, your bird will be on a good diet. Don’t give up, even if it seems to take months. You won’t fail.
After you have successfully converted your bird, you may, if you wish, leave some dry foods in the cage in the daytime.