Google only supports localization at the country level. You can have separate sites with the same content for two countries, even if they speak the same language. For example you can have separate sites for the US, UK, and Australia. Google allows this because sites often have to support several differences between countries:
Different word spellings
If your developer is using a 301 Redirect then this is the best practice.
Google guideline (and I guess guideline from other search engines) is simple on this matter. When you are moving a URL and want to maintain its SEO value implement a 301 Redirect when the change is permanent.