Super long post
When I got Rosie she was super under weight(50 grams underweight to be exact! a lot for a bird that weighs 280 grams healthy) from her organs failing combined with the inadequate nutrients of a sun flower seed staple diet that she had for four years, and a abusive neglectful home that probably didn't feed her much better for who knows how long.
First week I removed sun flower seeds from her diet completely, in which safflower became her favorite and she would pick it all out. I also made sure to give her fresh veggies everyday even though she rarely touched them exept to throw them on the floor.
I then bought the fiesta conure seed mix(galahs have little beaks) because it has colorful pellets mixed in with the seeds. I didn't care that it was like junk food, all I wanted was for her to try something other then seeds. I( also bought Zupreem natural pellets and mixed them in with her seeds.
2 bags of fiesta and zupreem later she started taking the fiesta "pellets" and nibbling on them, she also started putting them in her water dish to make them softer. I started adding more zupreem and also lafeabers into her seeds but she lost 5 grams so I added more seeds.
It's very important for a malnourished/underweight bird, never starve them to force them to eat a different food.
While still adding the pellets to her seed mix I added another bowel with a mixture of 3 pellets- Lafeabers, Zupreem Natural, and Roudybush pellets. It took a while(about $230 of uneaten pellets) but she eventually started to eat a little bit of the zupreem. I learned that Zupreem had sugar in their recipe so I slowly removed it from her food as well as putting more pellets into her seeds.
Now at about 5 months in she's eating a fair amount of lafeabers pellets along with her seeds but I can't seem to get her to stop eating seeds and she's still very underweight.
until the 6th month she is still eating seeds(although much less then before), for Christmas my friend gets her a
fiesta seed foraging box. Rosie pretty much ignores it for two weeks until I decide to giver a very small amount of seeds one day and show her the seeds inside the box. I then have to go somewhere for 2 nights and move her into my parents room.
When I get back her foraging treat is entirely eaten and she was suddenly eating pellets perfectly. I'm not really sure how it happened but after being gone those two days I was able to completely remove seeds from her diet. Also without seeds in her diet she starts eating some of her fresh veggies everyday(yay)
Now on a staple diet of lafeaber's pellets she slowly starts gaining weight and having more energy(she slept a lot before). I'm still worried about her though and my mom decides to let me take her to the vet for a check up(Yay).
This is where the major improvement happens, so the vet pretty much forces me to buy harrison's(which is fine, it's a great brand)but Rosie absolutely hates it because it's too hard and doesn't like it soaked. I go through three 1 pound bags and decide to try something else. Iorder a bag of Totally Organic pellets and after two weeks I remove Lafeabers and she loves totally organic because of how crumbly it is. She is now gaining weight really good and her feathers(she had a calcium deficiency) are looking better and better. She also has more energy then she's ever had and just seems much healthier and happier.
Since being completely on pellets she went from weighing 230 grams to 280 grams and now after a year and 1 month her calcium deficiency is almost gone too(restored her calcium naturally with dark leafy greens and other healthy veggies and by using almond pieces as treats.)
Rosie's avian vet told me that by feeding her a healthy diet her body will restore itself.
Hope that helps a little bit, it may take a while but it will be worth it in the end 