Hi Duilio,
This is as expected, We don't allow changing chart type without assigning a datasource to it.
There is a very good reason for this.
We validate the structure of the datasource and see if the selected chart type can be rendered for it.
E.g. Waterfall chart will not work if you add more than 1 Dimension in the rows.
You try to add a datasource and then the chart type should be changeable.
Regards,
Bharath