purging exernal data references in Shape Data

I have an org chart that is linked to an external data source.
I delink the external data source.
However, when I open the Shape Data window, there are fields from previously
linked data sources.
When I link to a new data source, the old Shape Data fields are not removed
and block my ability to format new linked data.
How do I purge the old Shape Data fields in the existing file?
To test this, I created a new org chart. None of the old Link Data fields
are present, and I can successfully format the new data on the new chart.
So ... after deleting a link to a data source, how to also remove the
references to those fields in Shape Data. IN other words, how to start
"fresh" with an existing visio org chart?