The ones that are not treated will continue to pass worm eggs and the ones that have been treated can be reinfected. I would treat all at the same time.
Or you can just eat the eggs. Many members do and are alive to tell you about it. Myself included. I don't, however, sell those eggs.
You can use the Panacur. The dose would be 0.5ml/Kg of body weight.
I weigh my birds on a digital scale I purchased for this purpose only.
Then I take piece of bread and squirt the dose into the bread and feed it to the bird in a room away from the other birds so the bread isn't stolen. I work...