Saving changes after table edit in SQL Server Management Studio
Go into Tools -> Options -> Designers-> Uncheck "Prevent saving changes that require table re-creation". Voila.
That happens because sometimes it is necessary to drop and recreate a table in order to change something. This can take a while, since all data must be copied to a temp table and then re-inserted in the new table. Since SQL Server by default doesn't trust you, you need to say "OK, I know what I'm doing, now let me do my work."
Tools | Options | Designers | Table and Database Designers
Uncheck the option "Prevent saving changes that require table re-creation".
To work around this problem, use SQL statements to make the changes to the metadata structure of a table.
This problem occurs when "Prevent saving changes that require table re-creation" option is enabled.
Source: Error message when you try to save a table in SQL Server 2008: "Saving changes is not permitted"