From Save (Not Permitted) Dialog Box on MSDN :

The Save (Not Permitted) dialog box warns you that saving changes is
not permitted because the changes you have made require the listed
tables to be dropped and re-created.

The following actions might require a table to be re-created:

  • Adding a new column to the middle of the table
  • Dropping a column
  • Changing column nullability
  • Changing the order of the columns
  • Changing the data type of a column <<<<

To change this option, on the Tools menu, click Options, expand
Designers, and then click Table and Database Designers.
Select or clear the Prevent saving changes that require the table to be
re-created
check box.

See Also
Colt Kwong Blog Entry:
Saving changes is not permitted in SQL 2008 Management Studio