SQL Server - Saving changes is not permitted (Alter Table)

Message:
 
Saving changes is not permitted. The changes that you have made require the following tables to be dropped and re-created. You have either made changes to a table that can't be re-created or enabled the option Prevent saving changes that require the table to be re-created.

 

Solution:

1. Open SQL Server Management Studio.
2. Clic Tools menu: Options.
3. In the navigation pane of the Options window, click Designers.
4. Select or clear the Prevent saving changes that require the table re-creation check box, and then click OK

More info: https://support.microsoft.com/en-us/kb/956176

SQL Server - How to generate auto number in sql server 2008 r2?

Right clic on table - Modify Table or Design.
Clic on the column (the autonumber defined) 
Change the Identity Column with the column name to autonumber column defined