I don’t think it can be done with a single t-sql command.

If you look at the script Management Studio creates when you change the ‘Is Identity’ property of an identity column, you will see that it needs to create a temporary table, drop the original table, and then rename the temporary table.

-CarlosDL

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There is no way to take an existing column and make it an identity column or to take an existing identity column and remove the identity property.

You need to create a new table with the identity flag set, then move all the data into that table.