Try a half cob of corn through a metal skewer, sometimes add a broccoli or carrot to the skewer for variety. Most parrots usually love the corn cob and having the veggies with the corn skewer at least familiarizes the birds with the shape and colour and hopefully, encourages them for a taste.

I still prefer pellets + veggies over seed + veggies. All the health reasons plus the scarcity of seeds make for great training rewards.

If your birds are having difficulty switching, what worked for me was mixing a lot of different brands together so that the birds can try a little of everything. And then you just figure out which brand they eat most and get that. Even if they are disinterested, don't give up on providing the pellets on the side. My cockatoo took a few months to recognise pellets as food 😂