While creating and editing a database in SQL server 2008 I soon ran into a brick wall, or a 'by design' feature of the Database Management Studio.
When you design a table in a database and then try to make a change to a table structure that requires the table to be recreated, the Database Management Studio will not allow you to save the changes.
This is caused by a configuration setting that default results in the following dialog:

Error dialog

This dialog states: "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."

You can only react clicking the Cancel button.
And than Management Studio shows this dialog:

So what you want to do to configure the setting with the value you want (IMHO should be the default setting!).
Go to: Tools > Options

Tools menu choose Options

Select the tab Designers and choose Tables and designers

And uncheck the option: "Prevent saving changes that require table re-creation".
Once you now save changes it will work.