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it was actually a challenge to try to detail out. It certainly seemed so And you are very welcome - glad to have been able to help. -Andy

I'm going to throw in a completely different option, because in my opinion, messing with this kind of multiple version situation is going to put your huge website at massive risk of screwed up rankings and lost traffic no matter how tricky you get. First, I'm assuming that significant high-level load testing has been done on the dev site already. If not, that's the place to start. (I'm suspecting a Joomla site for 40 million visits a month will have lots of load-balancing in place?) Since by all indications, the sites will be identical to the visitor, I'd suggest switching to the new site, but keeping the original site immediately available in near-line status. By setting the TTL of the DNS to a very short duration while in transition, the site could be switched back to the old version within a minute or two just by updating the DNS if something goes pear-shaped on the new site. Then, while the old site continues to serve visitors as it always has, devs can fix whatever issue was discovered on the new site. This would mean keeping both sites' content updated concurrently during the period of the changeover, but it sounds like you were going to have to do that anyway. There's also the small risk that some visitors would have cached DNS on their own computers and so might still get sent to the new site for a while even after the DNS had been set back to the old site, but I'd say that's a vastly smaller risk than screwing up the rankings of the whole site. Bottom line, there are plenty of load testing/quality assurance/server over-provisioning methods for making virtually certain the new site will be able to perform before going live. Having the backup site should be a very short term insurance, rather than a long term duplication process. That's my perspective, anyway, having done a number of large-site migrations (though certainly nothing approaching 40M visits/month) Paul Just for refernce, I was involved in helping after just such a major migration where the multiple sites did get indexed. It took nearly a year to rectify the situation and get the rankings/traffic/usability back in order

And in this case, "can't" is not only dev code, but marketing code also. They are so afraid of change here...but I get that, we are successful too. And just my saying "hey, we have the worst case scenario for SEO right now" is not always enough...they want more opinions. Which is why this Q&A feature of SEOMoz is awesome! Thanks again for your thoughtful responses.

Totally valid question. In this case, it's a matter of the content chasing the users and accepting that for the type of content I'm offering, the average user would likely prefer to stay inside of FB. But, as you point out, it's absolutely a non-zero risk, and domain juice is where I'm starting the risk-assessment process :).