The ability to change which client template to use has been added for v3.2 in CORE-961.
You'll always want to keep backups of core template files you modify, and merge them into new releases accordingly. Even if you were using custom templates, you would still need to check them on new releases since data, structure, and language definitions can be added, changed, or removed that has the potential to break things in your templates. The beta releases would be a good time to update your custom templates with these kinds of changes.