Cannot alter RichTextMode on a existing Pages Library with Publishing HTML column

I have provisioned a custom content type that has tab[1-5] site columns with this type of definition:

<Field Type="HTML"
     DisplayName="Tab 1 Content"
     RichText="TRUE"
     RichTextMode="ThemeHtml"
     Required="FALSE"
     Group="Custom"
     ID="{b4882caa-b734-4e31-a3fc-72c65f59178a}"
     StaticName="Tab1Content"
     Name="Tab1Content" />

This content type has been deployed and applied to a number of Pages libraries in the sub-sites. It turns out that we need to modify the RichTextMode to "FullHtml" because we want need to be able to use OOB embed feature and allow external embeds. Here is the code that i am trying to use to modify the site column definition and push the changes into the target sub-sites lists.

I keep getting the error:

Exception calling "Update" with "1" argument(s): "/en-us/Pages : Non-supported field type change. The field cannot be changed to the new type. Please check the new type and try again.

Same Error occurs when I attempt to manually modify the list's field definition as well through SchemaXml property setter:

$listField.SchemaXml = '<Field Type="HTML" DisplayName="Tab 1 Content" RichText="TRUE" RichTextMode="FullHtml" Required="FALSE" Group="Custom" ID="{b4882caa-b734-4e31-a3fc-72c65f59178a}" StaticName="Tab1Content" Name="Tab1Content" Customization="" SourceID="{dfd5f908-5c16-4488-a552-d681db8bf7b2}" ColName="ntext13" RowOrdinal="0"  RestrictedMode="TRUE" Version="3" />'
$listField.Update()