At the moment it means editing the template in both styles and copying and pasting from one into the other. And replicating this process for all the templates. It's going to take hours.
How much easier it would be - saving a whole bunch of time - if I could select the templates from the template list of the source style and copy them into the target style, overwriting the templates in the target style.
The process could, I guess, also be done by exporting selected templates and then importing them into a different style, overwriting only the imported templates. Same thing, different name for it.