In SQL Server 2008, when you change table from SQL Server management studio, if that change needs table to be dropped and re-created, it will return and error and change will not be affected. Error is shown in following image.
This will effect operations like changing int column property to identity and changing varchar column to int etc.. In case of table recreation, it can take lot of time if there is lot of records in the tables.
However, there is an option to allow these changes to occur.
Select Options from the Tools menu. In that dialog go to Designers option and de-select highlighted option. By default this option is selected.