I tried to edit the column types, and save it. But received the error message:
Saving changes is not permitted. The change you have made requires the following table 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.
To solve the problem, you can go to Tools–>options–>designers–>uncheck “Prevent Saving changes that require table re-creation”.