Generating update script in Visual Studio 2010 Premium using Schema Compare is dropping tables, rather then altering?

I'm using Visual Studio 2010 Premium's Schema Compare function, to compare two databases.

I have added columns, altered some columns and changed stored procedures. Visual Studio is picking the changes up fine, when I drill down into each table, the Update Action for existing tables is set to 'Update', and the Update Action for the new columns it set to 'Create'.

When I generate the update script it's actually dropping the tables, before creating them again.

How can I generate an update script tha开发者_JS百科t just alters the existing table?