Working with Tables in Database Diagram (Visual Database Tools)
You can modify and create database tables in either Table Designer or Database Diagram Designer. For information about working with tables in Table Designer see Working with Tables in Table Designer (Visual Database Tools).
If the table is published for replication, you must make schema changes using the Transact-SQL statement ALTER TABLE or SQL Server Management Objects (SMO). When schema changes are made using the Table Designer or the Database Diagram Designer, it attempts to drop and recreate the table. You cannot drop published objects, therefore the schema change will fail.