Microsoft Dynamics online: change field's data type

Question

What are the steps for changing Field's (not custom) data type? I go to Customizations -> Customize the System -> Entities -> Activity -> Fields -> Subject. I need to change Type from "Single Line of Text" to "Option set". How do I do that? I have Visual Studio 2012 and Microsoft Dynamics CRM SDK but I'm not experienced at it.

EDIT: Do I need to retrieve FormXml from SystemForm entity?

Answer 1

You can't change the type of the field after its creation (you can only change the properties, for example the minimum and maximum value for numeric types or the optionset entries).

If is a custom field you can delete and add again with the right type, if is a standard field the suggested way is to hide it from forms and views and create a new custom field instead.