Alter Table in SQL Server 2008:=> Gives me error

I have created CustomerTbl in SQL Server 2008.

For that i have,

Right Clicking the Table Folder in ServerExplorer and selecting Add New Table.

1. Now i Have Added three column say CustomerID, FirstName, LastName with datatype int,varchar(50),varchar(50) respectively.

2. Now save the table with CustomerTbl name.

3. now I reopen the Table Defination by RightClicking Table inside ServerExporer and Selecting OpenTableDefination.

4. Now i changing the Property of CustomerID column. I need to automatically increment the value by One each time new record 开发者_Python百科inserted in table. for that I set IsIdentity property to true. using selecting field and ColumnPropery Tab.

5. But Problem Occur when i try to save the table. I am getting following error box.

why i can't make any changes in table's fields property after we have created table and saved it once.???

Please help....

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It's a setting in SQL Server Managment Studio.

Menu Tools - Options - Designers - Prevent saving changes that require table re-creation