Hi, It is still the case that you can't switch an existing story between different Story Map app templates. So if you have created a Story Map Cascade, you can't switch it to be a Story Map Series. This applies to all the templates.
Some Story Map apps have different layout options that you can switch between once you have created your story. So you can change a Story Map Series to use that app template's Side Accordion, Bulleted or Tabbed layout at any time. Story Map Journal has two layout options you can choose between: Side Panel and Floating Panel, and so on.
You can re-use the data from one story map in another fairly easily, but it isn't automatic. So you can take the web maps you use in a Story Map Cascade and use them to create a Story Map Series. You could also take the places defined for a Story Map Tour and use them in a Story Map Shortlist.
If you are specifically interested in making your Cascade present more of a tabbed UI, you could add bookmarks in to the header of the Cascade that take people directly to particular sections in your Cascade.
You can also embed existing stories in new stories. For example suppose you have created a Story Map Journal and you'd like to present it as a Story Map Series using the Tabbed layout instead. You could create your Story Map Series using the same maps, media and narrative that you used for your Story Map Journal. But you also have the option of adding that Story Map Journal into a tab in a Story Map Series, and then adding other content for the other tabs. Similarly, if you have a Cascade app and want to include content presented as tabs, you could create a Story Map Series containing that content, and then embed it into your Cascade. See this FAQ for more info about embedding stories in stories.
We do have to long term plans to possibly make switching between different formats possible, so let us know if this is an appealing enhancement.
Rupert
