Converting a column from nullable to non-nullable is explicitly
checked for and disallowed.  I'm not 100% sure why, I suppose
it's because there would be no real good way of dealing with
an existing NULL at runtime.  I think I would prefer to see
it allowed, and convert existing NULL to default;  however
this is arguably less "correct".

I guess you will have to drop and re-add the column.
If the table involved doesn't have a unique key, you could
temporarily add a holding column to that table and update
it with the old column's values while you drop and recreate
the old column.

Karl