update new field to a default value on old records

update new field to a default value on old records

Hi,
 
I would like to add a new field to my forms and automatically update it to a default value on my old, previously created records.
 
I have 100 clients, and I would like to add a flag field on all of them to a default value of "NO",
retroactively to all of my clients.
I know there is a way to do so without a loop - by using the "required field" functionality.
Somehow it worked for me a few months ago, and the fields were well updated.
I don't know why, it doesn't work anymore.
 
 
How do I do that?