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 recreated or enabled the option Prevent saving changes that require the table to be re-created.

Enabling the option to allow you to complete these types of actions is pretty easy, from the "Options" menu, select the "Designers" tab, and the uncheck the "Prevent saving changes that require table re-creation". Then click "OK"

Problem solved. Hey Microsoft: It would be great if this feature would allow for three levels: On, Off, and Warn.