modify_text_entity

I have a document that is opened as read only. I want to change a character entity to display something differently than what is declared. I set rochange=true to be able to change the document, and then try the modify_text_entity command but it comes back with an error sayinig that the document is read only even though I am able to edit the document. This used to work in 4.3, but seems to be broken in 5.3. Anyone have any suggestions on getting around this limitation. Thanks.