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Re-launch s directly proportional to traffic loss but proper planning and launching strategy can help you keep this drop to minimum. If you are relaunching with keeping everything document, redirections, 404, content updates and more than this temporary dip can be minimum for a small amount of time. Whenever relaunching it is important to keep few things in mind -          Get a PR for the relaunch -          Get new quality links back to the profile -          Update new content more actively. This will give Google a hint that you are serious about your business and people wants to see you more in SERP results. Hope this helps!

Your dev is correct. You do need to set up redirect. We have a great guide about how to set up redirects. Redirects that are set up properly will help your SEO, not hurt it.

This is really great, I appreciate your help