This is a very annoying and confusing error when you first start using SQL 2008. It is caused by SSMS itself, and not SQL.  To make changes to tables through the designer, follow these steps...
<ul>
	<li>Open SQL Management Studio</li>
	<li>Go to Tools &gt; Options</li>
	<li>On the Options window, click on Designers in the left treeview</li>
	<li>Uncheck 'Prevent saving changes that require table re-creation'</li>
	<li>Click OK</li>
</ul>
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