I don't believe you can change the Allow Null values setting without recreating the field. You can not add a new field with Allow Null values set to
No on a feature class that contains records.
In response to your comments, your idea to disallow Nulls and have a domain will still not force users to input a value. If they don't pick a value from the Domain, it will just be an empty string
"" instead of a
To test I created a new feature class in my database (running on Microsoft SQL Express). The first field
MyField1 disallows Nulls, and has a domain. The second field
MyField2 disallows Nulls and has no domain, and third
MyField3 allows Nulls and also has no domain.
I created three points, and left some of the fields blank. I was able to do this on all three fields, it doesn't force me to input on any field.
As you can see when I use the Identify tool on the points,
MyField1 is blank (not
Null - both of these fields I left blank when I added the point, it honoured the No Nulls by saving an empty string.
Even if you set the attribute window to pop up on feature creation (Editor > Options > Attributes), it still won't force you to enter any values, so Nulls/Blank values are still a possibility.
When Display attributes dialog before storing new features as enabled, it pops the attributes window up as soon as I create the point, but I can just click on OK to close it.