Increase speed altering column on large table to NON NULL
Instead of changing the column definition you could add a CHECK CONSTRAINT that doesn't allow NULLs for that column. The table will still need to be scanned but it won't need to modify every single data page, so it should be a much faster operation. Regrettably, a Sch-M lock will still be held during the operation. One trick is to try to get as much of the table into the buffer pool as possible before attempting to add the constraint. That may reduce the amount of time the Sch-M lock is held.

You can then remove the constraint and change the column definition during your next maintenance window.