Feed old and new side by side in the group's own spot
Corydoras learn a new diet by grazing across it among food they already trust, so the changeover happens on the substrate, in the patch where the group always feeds. For the first several evenings, drop the usual sinking food with a few pieces of the new food mixed through it. Every few days, tilt the balance — fewer old pellets, more new — until after a week or two the ration is entirely the new food. Corys feed by smell and by mouthing what they find, and a new pellet sitting beside a familiar one gets investigated in passing; a whole meal of strangers can get investigated by nobody, especially in a group that startles easily.
Time the drop for when corydoras actually feed
The stage of the changeover matters less than the audience being present for it. Drop the mixed ration after the main lights dim and the day-active fish have settled, because corydoras feed most confidently in the evening and a new food eaten under pressure from faster tankmates teaches nothing except haste. If your community fish still intercept the sinking food, feed them a small distraction at the surface at the far end of the tank while the corys' mix sinks at the near end. A new food that keeps being stolen before the group reaches it will look rejected when it was never actually tasted — check that the corys, specifically, are the ones eating before you judge the recipe.
Let the morning substrate grade each stage
Judge every step of a corydoras diet change by what the substrate shows you next morning. Old food gone and new food gone means advance the mix. Old food gone and new pellets sitting swollen and untouched means the group has voted — hold the ratio where it is for several more days, or prepare the new food differently: many sinking pellets are taken more readily after a brief soak in tank water softens them to the texture corys prefer to mouth. New and old both left is a different message entirely; a group that stops finishing even its familiar food during a changeover may be reacting to water or illness, and that possibility outranks the feeding plan until it is checked.
Protect the shyest fish in the group
In any corydoras group there is a boldness order, and the fish at the back of it meet new food last. During a changeover, scatter the mixed ration across two or three spots in the group's territory rather than one pile, so the hesitant fish find pieces away from the scrum and get the same gradual education as the bold ones. Watch that the smallest corys are still rounding out and foraging with the group each evening. A single fish that never joins the new feeding spots, grows thin, or sits apart with clamped fins while the rest transition happily is a fish to look at closely, not a slow learner to wait out indefinitely.
When to stop the changeover and look wider
Halt the diet change and investigate if the corydoras group shows rapid breathing, worn barbels, loss of appetite for all foods, or deaths during the transition — a new food is a mild stress, and it can be the thing that reveals a tank already struggling rather than the cause of the struggle. Test the water, siphon the substrate, and seek an aquatic veterinarian's or experienced shop's help if the signs persist on the old diet. A paused changeover costs nothing; corys can live on the old food indefinitely while you sort the tank out. For the orderly version, KinsLe Care Teams record your corydoras' current mix as the assigned evening duty — stage, amount, and feeder logged with a timestamp when done — keeping the gradual shift on rails across the whole household.