The changes you have made require the following tables to be dropped and re-created
When changing tables in SQL Server Management Studio 2008 you may get the following error: Saving changes is not permitted. The changes you have made require the following tables to be dropped and re-created. I was surprised when I saw this message first but there is very simple solution.
From top menu select Tools and then Options. Select Designer and Table and Database Designers.
Uncheck the box Prevent saving changes that require table re-creation. Now you can edit your tables without being stopped by re-creation limits.
Update. As mxmissile pointed out in his comment then don't use this on tables with millions of rows.