title: Alter column to be identity
body: <p>I've already read the following answers about the impossibility to alter a column into identity once has been created as a regular int.</p>
  
  <p><a href="https://stackoverflow.com/questions/1049210/adding-an-identity-to-an-existing-column">Adding an identity to an existing column</a></p>
  
  <p><a href="https://stackoverflow.com/questions/17291680/how-to-alter-a-table-for-identity-specification-is-identity-sql-server">How to Alter a table for Identity Specification is identity SQL Server</a></p>
  
  <p><a href="https://social.msdn.microsoft.com/Forums/sqlserver/en-US/04d69ee6-d4f5-4f8f-a115-d89f7bcbc032/how-to-alter-column-to-identity11?forum=transactsql" rel="nofollow noreferrer">How to alter column to identity(1,1)</a></p>
  
  <p>But the thing is I have a table which has been migrated to a new one where the ID was not declared as identity from the beginning, because the old table which was created with an ID identity a long time ago has missing rows due to a purge of historical data. So as far as I know, if I add a new column as identity on my new table, it will automatically create the column sequentially and I need to preserve the IDs from the old table as-is because there is already data linked to these previous IDs.</p>
  
  <p>How can I do transform my ID column from the new table as identity but not sequentially, but with the IDs from the old table?</p>