How to remove IDENTITY from a Column of a SQL Server?
The one line answer is – There is no specific way for disabling the IDENTITY on a Table Column.
There are few hacky ways to do this so let me share here.
Option 1:
  • Create a replica blank table without any constraint
  • Transfer all data from your main table to replica table
  • Apply proper constraints like a Primary key or Unique key on that column which is identity column of the main table
  • Dropped the main table
  • Rename your replica table to the original table name

Option 2:
  • Create a replica column in the same table
  • Update new column by a value of IDENTITY Column
  • Check the reference of IDENTITY Column and If any then remove temporary
  • Drop the IDENTITY Column
  • Rename the replica column to the original column name

Ref:  https://www.dbrnd.com/2018/03/sql-server-how-to-remove-identity-from-a-column/

How to set IDENTITY to existing column of a Table?
Actually, there is no any specific option for setting IDENTITY to existing Column of a Table.
But there are different tricks to do this, sharing one of the tricks here…
Create a sample table without identity column:
Check the data:
Add new IDENTITY Column to table:
Drop old ID Column:
Rename new column to old column name:
Result:
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SELECT *from tbl_TestIdentity
 
Name       ID
---------- -----------
ABC        1
XYZ        2
PQR        3
WER        4
JKL        5
ZXC        6