I made some changes after suggestions from BCdotWEB

For the class: DebenturChangesRepository I changed the name of CreateForChangedCalledIn to CreateForNewDebenturs so that if I need to monitor additional fields than I only need to change my repository.

I change the sql to use Dapper which shortens the code by aprox 50%!.

Here the new code:

public class DebenturChangesRepository : IDebenturChangesRepository
{
    private readonly string connectionString;

    public DebenturChangesRepository(string connectionString)
    {
        if (connectionString == null) throw new ArgumentNullException("connectionString");
        this.connectionString = connectionString;
    }

    public List<DebenturChange> CreateForNewDebenturs()
    {
        var sql = "Select ClaimNo, CalledIn " +
            "From Debentur " +
            "Where ClaimNo Not In (Select ClaimNo From DebenturChange) ";

        using (var connection = new SqlConnection(connectionString))
        {
            return connection.Query<DebenturChange>(sql).ToList();
        }
    }

    public List<DebenturChange> CreateForChangedDebenturs()
    {
        var sql = "Select d.ClaimNo, d.CalledIn " +
            "From Debentur d join DebenturChange c on d.ClaimNo = c.ClaimNo " +
            "Where IsNull(d.CalledIn, 0) <> IsNull(c.CalledIn, 0) ";

        using (var connection = new SqlConnection(connectionString))
        {
            return connection.Query<DebenturChange>(sql).ToList();
        }
    }

    public void InsertDebenturChange(DebenturChange debenturChange)
    {
        var sql = "Insert Into DebenturChange (ClaimNo, CalledIn) " +
            "Values (@ClaimNo, @CalledIn)";

        using (var connection = new SqlConnection(connectionString))
        {
            connection.Execute(sql, debenturChange);
        }
    }

    public void UpdateDebenturChange(DebenturChange debenturChange)
    {
        var sql = "Update DebenturChange " +
            "Set CalledIn = @CalledIn " +
            "Where ClaimNo = @ClaimNo";

        using (var connection = new SqlConnection(connectionString))
        {
            connection.Execute(sql, debenturChange);
        }
    }
}