If it's a total overhaul with new URLs, then you MUST use 301 Permanent Redirects for all the old URLs. However, if it is a features overhaul, I'd update gradually to ensure your rankings don't drop.
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RE: When launching a site redesign, what is the best way to roll it out?
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RE: Start SEO for new domain with no rank
From my experience, I would start to create and add some valued content on your site. Register a Twitter/Facebook account relating to your website, and post any site updates/new pages via these two channels.
In addition, create a sitemap (keep it up to date), and submit via the Google Webmaster Tools console. Your site will soon become indexed in Google. Your next step and more harder step is to gain valuable links to your site.
Regarding the targeting of long tail keywords, since this is much easier - this is something I'd suggest you try first.
Good luck !