In SQL server 2008 modifying table is not permitted by default. From the table designer if you try to modify a table it will display a message :
"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.
All you can do is Cancel it.
Solution:
Go to Tools > Options > Designers > Table and Database Designers :: Form the left pane uncheck the option " Prevent saving changes that require table re-creation. > OK
