Hello there !
Budgies are quite difficult birds when it comes to changing diets. Budgies not used to fresh food will not even acknowledge it let alone give it a try.My budgies were exactly the same as yours a few years ago.
You should begin with small steps. I've found that they show a preference towards green coloured foods.
A few leaves of spinach , kale or some broccoli might make them curious enough to take a nibble.
But all my birds (budgies, tiels , bourkes, red rumps etc...) show a special love for Coriander (Cilantro)
. It works wonders during training. I have had completely wild budgies accept Coriander directly from my hands.
I highly recommend you give it a try !
Once you get them to eat one thing , they do become more open towards other foods as well. You can then move on to other fruits/veggies. Bell peppers , grapes , carrots , Mung sprouts are some of my birds' favourites .
You could also try sprinkling seed on some of the food to encourage them to have a try .
Budgies are flock birds. If you have one bird which readily accepts fresh food , its only a matter of time before the rest of your birds start accepting it too it
Good luck !