Convert Long Text to Rich Text

Using MDL, you can migrate custom Long Text fields to Rich Text fields. This is a permanent, one-way migration that cannot be undone.

The long text value is converted as plain text, even if the long text field contains valid HTML. For your new rich text field to render HTML markup, you must re-enter the value.

ALTER Object object_name__c (
    MODIFY Field field_name__c(
        type('RichText')
    )
);

Long text fields used in reference lookups cannot be converted to rich text.