Just build the whole site on another IP address so you can test straight online. The 301's are a good idea too, to not lose the visitors that find the old pages in the SERP's + they help the search engines find the new pages quickly.

If the whole thing is optimized, then what you will see is an increase in visitors from the day that you put the new site online. (We do this a couple of times a week and it goes like this in 99% of the cases.) Unlikely that you'll see a reduction in the number of visitors.

However, if you start monitoring for a couple of specific keywords, then you might get dissapointed. You have to look at over all traffic, not a a couple of specific keywords.

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Otimização de Sites (SEO in Brazil)