Changing the type of a property (for example from StringProperty to XMLProperty) is only sensible when the property contains no content. However, you can not simply change the type of an existing property. Instead, you have to delete the property (see Section 4.3.9, “Deleting Properties”) and afterwards introduce the new property (see Section 4.3.6, “Adding Properties” or use aspects to add properties, see Section 4.2.3, “Attaching Properties to Existing Content Types”) into the content type model.