Of course it is a choice to just add a gender value and an indicator it is a value to be double checked.

Updating the current column content isn't such a big deal, handling the consequences of your choices is !

IMHO you would be better of leaving the NULL or adding an Unknown value for that column and restrict it all by a constraint !

update yourtable

set gender = 'U' /* U = unknown or just set it to NULL so there is no doubt it is unknown */

where gender is null

or gender not in ( 'M', 'F', 'U' ) ;

/* after fixing the content, avoid future faults */

alter table yourtable

alter column gender char(1) not null default 'U';

/* set gender constraints to avoid future messed up data */

ALTER TABLE dbo.x ADD CONSTRAINT

CK_gender CHECK (gender in ('M', 'F', 'U'))

;

Keep in mind, especially the last two actions may have consequences towards your current applications !

Test it - test IT