Saving Changes is not Permitted - SQL Server 2008

This error message is presented by SQL Server Management Studio only when you try to alter a table by:

1. Changing a column, i.e., null to not null
2. Changing data type on an existing column
3. Adding new columns

To avoid this error and let SQL Server Management Studio alter the table on fly, follow the following steps:

1. Click menu option Tools -> Options 
2. Select Designers from left pane
3. Uncheck "Prevent saving changes that require table re-creation" option
4. Click OK

Now try changing a table and then hit Save button. You should not see the message again!