When the user selects any of the RDF Node types (index, column, constant) from another types, change the expression to the default expression of that type unless it's changing back to the original type. If changing back to the original, restore the original expression.
Also, consider column expressions may be compatible between columns, so preserve a column expression if just changing columns.
For index types, a change between record and row mode already does this:
Changes between "row.record.index" and "row.index".
When the user selects any of the RDF Node types (index, column, constant) from another types, change the expression to the default expression of that type unless it's changing back to the original type. If changing back to the original, restore the original expression.
Also, consider column expressions may be compatible between columns, so preserve a column expression if just changing columns.
For index types, a change between record and row mode already does this: