Starnge Issue : Convert Single Line of Text To Multiple line of text

I am facing a very strange issue, there is a single line of text column in SharePoint List. I want to change it to Multiple Line of text, when i go to list settings and click on the column it does not give me options to change the Column, see the screenshot attached.

I then tried to change it using the code, Below is my code.

`

                        SPField field = fields[columnName];

                        try
                        {
                            if (field != null)
                                Console.WriteLine("Field Type current :" + field.Type);
                            field.SchemaXml = field.SchemaXml.Replace("Text", "Note");
                            field.Update();
                            NewsLetter.Update();
                            Console.WriteLine("Completed");
                            Console.ReadLine();
                            oWeb.AllowUnsafeUpdates = false;
                        }
                        catch (Exception ex)
                        {
                            Console.WriteLine("Message:" + ex.Message);                             
                        }

                    }`

I tried to run the code for my dev env and it worked the same code does not works for my prod env, where i need to make this changes. Can anyone please help this is urgent.