You don't "change" anything. The S-Corp is closed permanently and forever with a "final" 1120-S and final K-1's are issued to all owners. Then a "new" business that is a C-Corp is opened one day after the S-Corp is closed.
Now you "REALLY" have to know what you're doing to get dates right too. Otherwise you'll have major, major, major issues with transferring assets and inventory.
