Then here's how to stop this showing:
1) Open 'Microsoft SQL Server Management Studio'
2) Click the 'Tools' menu and select 'Options'
3) Expand the column on the left title 'Designers' and click once on 'Table and Database Designers'.
4) Untick the box labeled 'Prevent saving changes that require table re-creation'
5) Click 'OK'
6) Try amending your table again
7) You may get a warning saying 'Warnings were encountered during the pre-save validation process, and might result in a failure during save. Do you want to continue attempting to save?'
8) Before you click yes, make sure you won't lose any data by making a change such as reduring the number of characters accepted or by changing the field type from say a char(255) column to a datetime one, or something simular.
9) If you're happy, click 'Yes'
All should be working fine and as it should now.