This is a known, documented issue with SQL Server 2008 Management Studio, but certain DDL operations like ALTERing a column datatype from Management Studio fails.

For example, in Object Explorer, navigate to a table column > right-click on column > Modify. Then, change column datatype or length, then save and this error message displays:

To workaround this problem, go to Query Editor and issue the following DDL statement instead:

 TABLE dbo.FTPFile ALTER COLUMN CmdLine VARCHAR (100) ;

ALTER

GO

The column change is successfuly applied now.