Well, I'd go ahead & finish out the Corid (5-day treatment, I assume?) since you've started. If it was me, I don't think I'd add too much change all at once. But, it's hard to know without seeing them in person.
One thing of note: Cocci is found in dampness...and "damp" is the cause of SO many chicken illnesses. So, dry, dry, dry is your friend with your chickens. Keeping the coop as dry as possible, and keeping it clean, it the very best first line of illness defense. Overcrowding can also cause cocci to come on. So, make sure they have enough room.
If they are eating and losing weight, there's definitely somthing going on, though.
One thing of note: Cocci is found in dampness...and "damp" is the cause of SO many chicken illnesses. So, dry, dry, dry is your friend with your chickens. Keeping the coop as dry as possible, and keeping it clean, it the very best first line of illness defense. Overcrowding can also cause cocci to come on. So, make sure they have enough room.
If they are eating and losing weight, there's definitely somthing going on, though.