Can't change datasource after duplicating metric row in mixed mode

Using Grafana v4.7.0-pre1 (commit: 67e352b)

When using datasource --Mixed–, I am wondering why the datasource for a metric row cannot be edited.

For example I have an existing row and I use the Duplicate feature. The new row has the same datasource. But I want to change the new row to use a new datasource.

My goal is to have a single graph visualizing data from multiple datasources. If I create a new row from scratch, I can edit the data source for that row, but manually copying all attributes of the original row is not easy.

Here is a screenshot:
Screenshot from 2017-12-15 09-01-20

No, really good reason. It just has not been implemented and sounds tricky to implement in a way that makes sense. For example, if I changed a datasource from Graphite to Elasticsearch that would be strange.