If the maximum size of a row in your table exceeds 4,009 bytes, you cannot use an ALTER
statement to either change a column to SPARSE or add an additional SPARSE column.
During the ALTER, each row is recomputed by writing a second copy of the row on the
same data page. Because two copies of a row that exceed 4,009 bytes would exceed the
8,018 bytes allowed per page, the ALTER TABLE statement fails.
To avoid this
1) Create a new table, copy all the data to the new table, drop the old table, and
then rename the newly created table
2) Export the data, truncate the existing table, make the changes, and import the data back into the table.