No expert, but I switched mine because the first food they were on had bits of corn/etc in it they were picking out and leaving the rest.
New food was a pellet, so I crushed them in a coffee grinder at first and cut it 50/50 with the old food for a few days, then eventually switched to only the new pellets but still mostly crushed with the grinder (I tried not to pulverize it so there were still some larger chunks). Now its just the pellets and they seem to have accepted it just fine, just took time.