Yes: currently you must use [Field(NullableOnUpgrade = true)] property to ensure this.

Note: this part of API is preliminary - probably, we'll convert this to hint later.

If this property is used, DO considers the field is nullable until the final stage of upgrade. So e.g. you can use the following code in upgrade handlers:

public override void OnUpgrade()
{
    var session = Session.Demand();
    var none = new Person {Name = "None"}; // Replacement for former NULL value
    none.Friend = none;

    ( from p in Session.Demand().Query.All<Person>()
      where p.Friend==null
      select p
    ).ForEach(p => { p.Friend = none; });
}