altering a populated column from int to bigint

Hi , we run std 2008 and would like to change a non pk , non indexed column from int to bigint.  The column is already populated with int values and nulls.   

What impact would using the alter statement have on the size of the db?  If I already started the alter, what would happen if I cancel the query?  Would it have been smarter to unload the data somewhere and reload it into a redefined table?

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