In SQL Server 2008 Designer if you receive the following error "Saving changes is not permitted. The changes 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."

The fix is in Management Studio, go to tools -> options then uncheck "Prevent saving changes that require table re-creation". See image below.

http://pragmaticworks.com/community/blogs/brianknight/archive/2008/06/04/sql-server-2008-designer-behavior-change-saving-changes-not-permitted.aspx