How to add Identity in Existing database table column, without dropping the column.

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SQL-Server-2005

SQL-Server

I hv a database table book_ticket with columns tkt_no as primary key,
Now i want to autoincrement for that column without dropping the same column as it it referenced to other tables.
this is my table script..

CREATE TABLE book_ticket 
(
tkt_no  int  PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

plz help me out. thanks in advance.