So you are geting this warning ?
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.
Solution: Just uncheck the selected option. yes, it solves the problem.
Path: ==>SQL Server 2008 Management Studio ==> Tools ==> Options (then Designers).