Nevermind - I didn't look hard enough. Found this nugget of wisdom from Lowell...
for reference, you cannot alter a column to turn on the identity property once it has been created. you have to rebuild the table, which is exactly what the gui would do behind the scenes.
the GUI will do the following:
· build a new table to the correct specification
· migrate the data fromt he old table to the new.
· drop all the constraints against the original table
· recreate all the constraints against the new table
· drop the original table
· rename the new table to the original name