Hello! Welcome to the forum! I'd love too see pictures of your quaker and cockatiel! An easy way to add pictures is to click go advanced button under text box click the paperclip choose pictures then on far right click upload button, then close and post.
For diet changes you want to be careful, best done slowly. And best is to get a digital kitchen gram scale and track weights. Keeping track of weight is a great thing to do and will often help you catch health issues before they become life threatening.
There are several methods fir changing diet. And other will likely give you good advice and links.
My way is not the only way, and im not even going to claim it is the best way either. . But it has worked fir so many parrots in house that I will share with you.
I go the abundance route. I don't limit their current diet, I just build on it. Firstly offering leafy greens, vegetables, and much lesser amount of fruit promotes great health. I find offering the fresh stuff in wide shallow dishes like a casserole or serving dish, helps! I also think serving outside of the cage and letting them explore the food on their own speeds up the process. For leafy stiff like romaine lettuce, Swiss chard, ect I find hanging it helps, gets them nibbling.
For other veggies my 3 quaker parrots like chunks that they can pick up with feet. If its a green beans or snap peas I offer whole. For fresh corn on the cob I slice some kernels off and offer a hunk on the cob. Red chilli peppers with seeds and all is a real favorite, I just slice them in half. All peppers are a great source of vitamin A, which is much needed , and not found in seeds.
Pellets I offer in seperate dishes. Same for seed mix.
I find offering foods by hand several times a day is great introduction to new foods. I start out by offering seeds, till they learn I offer yummy stuff. Then I offer a new food thst has a high acceptance rate, like a part slice of spple, and I keep offering different stuff sll the time. I don't force them to take if tgey aren't interested i just plop in the dish. Over time my burfs have learned and are willing to come running to see what I have. Anything I eat thst is parrot safe I offer them a taste. Once birds start trying new stuff , the more willing they are to try new stuff. It sort of builds. If I manage to find something new and safe I have never offered before, they are more than willing to give it a chomp taste at first offering. But in the beginning you may have to offer over and over till one day rgey love it n