How is a column changed from sparse to nonsparse or nonsparse to sparse

• When a column changed from sparse to nonsparse or nonsparse to sparse, the storage format of the column is changed. This is accomplished by the SQL SERVER Database Engine using the following procedure:
• The SQL SERVER Database Engine adds a new column to the table in the new storage size and format.
• For each row in the table, the SQL SERVER Database Engine updates and copies the value stored in the old column to the new column.
• The SQL SERVER Database Engine then removes the old column from the table schema.
• It then rebuilds the table (if there is no clustered index) or rebuilds the clustered index to reclaim space used by the old column.