The best route for designing editable template is to redesign the templates directly to be the editable template. You can check the document  as a reference for creating editable templates.
We recommend to create the page templates from scratch. This is the best route and what we suggest to go with to avoid downstream issues and unintended side-effects.
It is advised to upgrade the templates and the general process is:
The basic idea is that all the components that are declared inside the static template should become a component declared as a content structure under the "structure" child node of an editable template. All synthetic resources should also become an actual node of the "structure" aspect of the template or be relocated in the HTL template or JSP page of an instanced component.