ALTER COLUMN on a text or ntext field

Hi,
 
I'd like to run the following command:
 
    ALTER TABLE cal_respurpose ALTER COLUMN [purpose] varchar(255)
 
but it falls over because the current [purpose] column is 'text': -
 
Server: Msg 4928, Level 16, State 1, Line 1
Cannot alter column 'purpose' because it is 'text'.
 
I can change it through design view in Enterprise manager, after clicking ok on the warning message, but I need to find a way to override this error in Script.
 
Is there a way I can overide the fact that it is a text column and change it to varchar?
 
Thanks in advance!
 
Paul