How can we do that?

//Source class
[Serializable]
 [HierarchyRoot]
 public class Client : Entity
 {
  [Field, Key]
  public int Id { get; set; }

  [Field]
  public string Name { get; set; }

[...]
  [Field]
  public DateTime RegistrationDate { get; private set; }

  [Field]
  public DateTime BirthDate { get; set; }
[...]
}

//source class, that uses Client

 [HierarchyRoot()]
 [Serializable]
 public class ProductCheckAction : Entity
 {
  [Field, Key]
  public int Id { get; set; }
[...]
  [Field]
  public ProductCheckFields Fields { get; set; }
[...]
}

 public class ProductCheckFields : Structure
 {
  [Field]
  public Client Client { get; set; }
[...]
 }

===
//new Superclass
 [Serializable]
    [HierarchyRoot(InheritanceSchema = InheritanceSchema.ConcreteTable)]
    public abstract class Contractor : Entity
    {
        [Field, Key]
        public int Id { get; set; }

        [Field]
        public string Name { get; set; }
    }

//modified Client class
[Serializable]
 public class Client : Contractor
 {
     [Field]
  public DateTime RegistrationDate { get; private set; }

     [Field]
  public DateTime BirthDate { get; set; }
[...]
}

===

===
//upgdade hints
hints.Add(new MoveFieldHint(typeof (Client), "Id", typeof (Contractor)));
hints.Add(new MoveFieldHint(typeof(Client), "Name", typeof(Contractor)));
===

But DO delete all clients at first, then we get error:

Xtensive.Orm.ReferentialConstraintViolationException: SQL error occured.

Storage error details 'Entity: Client; Field: Id (FieldInfo);'

ALTER TABLE [dbo].[ProductCheckAction] ADD CONSTRAINT [FK_ProductCheckAction_Fields.Client_Client] FOREIGN KEY ([Fields.Client]) REFERENCES [dbo].[Client] ([Id]);

Original message 'The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK_ProductCheckAction_Fields.Client_Client". The conflict occurred in database "shop", table "dbo.Client", column 'Id'.' ---> System.Data.SqlClient.SqlException: The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK_ProductCheckAction_Fields.Client_Client". The conflict occurred in database "shop", table "dbo.Client", column 'Id'.

update:

There is similar error in some cases (for example, when simply add one reference) and doing PerformSafely:

UPDATE [dbo].[Debenture] SET [Supplier.Id] = DEFAULT WHERE EXISTS (SELECT [aProductSupplier].[Id] FROM [dbo].[ProductSupplier] [aProductSupplier] WHERE ([aProductSupplier].[Id] = [Debenture].[Supplier.Id]));

DELETE FROM [dbo].[ProductSupplier];'
Original message 'The DELETE statement conflicted with the REFERENCE constraint "FK_PrimaryIncomingWaybillAction_Fields.Supplier_ProductSupplier". The conflict occurred in database "Drezex_B_Major", table "dbo.PrimaryIncomingWaybillAction", column 'Fields.Supplier'.
The statement has been terminated.'

DO deletes everything from ProductSupplier and leaves database broken.

Very strange: why PerformSafely makes queries that can DELETE all data from some table? I think, it is not safely action :)