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1. Plan your changes well. Have an outline of what the final site will be, so you can see the difference.
2. Do not start moving anything yet, but set up the new domain name, with sections to match the existing site - ie identical navigation, but with just key sections in place. Insert brief, but relevant introductory tcontent, and provide one quality link to the new site.
3. Give the new site time to be 'assimilated' into the SEs knowledge of your new site.
4. Set up a 301 from the old to the new, and move all content to the new.
5. Add new content, but do NOT do any major structural or navigational changes to the new site. Check navigation as you go; submit a new sitemap after each update.
6. Be sure you have a useful, reader friendly 404 page.
7. If you have to do any changes associated with the move, make the changes over a period of weeks if you can, so that the 'new' is progressively assimilated at each spidering. If you can leave some of the site untouched throughout, and change that next year, that will help. AVOID any major structural or navigational changes to the new site.
8. Once the changes are done, be sure you have a 301 from non-www to www; check that there are no links to 'index' - all internal 'homepage' links should go to '/'
9. Start a new program of link building, and get people to revise old links where you can.
10. Check it all one more time - and keep your fingers crossed.
In quick response to your points:
1. Sites will look identical
2. Same as 1.
3. How much time do you think it would take for content to be "assimilated"?
4. OK
5. Not planning any structural changes, and every page on the old site would 301 to the new. Ok re sitempas.
6. Done
7. There would be no changes specific to the move.
8. OK
9. OK
10. That's my worry... Don't want to leave much to chance!
1. So I'd set up the new site a few months before the move. There's never a 'guarantee' against the sandbox, but correct 301s, smooth change and stability appear to be the key points.
2. The only way to avoid duplicate content issues is not to have duplicate content; specially for interlinked sites (which is risky, and more so as the number of sites increases). But duplicate content does not cause a penalty, merely possible supplementary listings, and occasionally (if identical meta tags etc., delisting. Once the copy becomes unique through later removal of the dupes, things will self-correct.
3. There's no way better than a 301, that's for sure. But there will be some damage; you will need to start looking to convert as many links as possible to the new site, especially directories and other key links. And get some new ones. Plus keep adding (not changing) to the new site.
4. Be sure your navigation is always up to speed - xenu is your friend.
From what you've said, I'd have two concerns; duplicate content issues and interlinking. For the interlinking, I'd minimise it, and favour those to the new site. Now would be a good time to review all your outgoing links, so's not to risk upsetting Google at a sensitive time!