I'm often asked by clients and colleagues about the potential pitfalls around SEO when it comes to site migration. Here I share a few things to consider.
Primary concern should be at least maintaining (and preferably improving) the commercial benefit of the site, be that revenue, sales, downloads, or whatever other metric you can measure.
Inevitably this type of conversion is as a result of a certain number of visits from search engines, which are obviously born off visibility in search engines, so knowing what your most important keywords are and benchmarking them before you start changing the site is key.
Once you have your keywords run a ranking report on them so that you can see not only where you rank for them but also which URLs are ranking. There are a number of tools to do this but many SEO's believe that the most accurate way is to do this manually. Make sure that you are not logged in to your Google account when you are doing this or you will see personalised results.
Next make sure that you have like-for-like content on the new site to maintain or improve these rankings.
Now you have a list of URLs ranking in Google extend the list by setting up a Google Webmaster Tools account (or logging in to your existing one) and find out which URLs you have back-links pointing at.
These two lists are the most important URLs for you to manage. Either plan to keep them for the new site or plan to map them carefully using the correct type of redirect to the most similar page on the new site. Always use a permanent (301) redirect at page level to do this. It's always also a good idea to have a catch-all redirect rule for any URLs you may have missed, and a custom 404 page as a fail safe (this is a website best practice anyway).
If you are changing URLs and you have a lot of high domain strength back links (see SEO Moz's open Site Explorer for more on this) it is also worth contacting each of those sites as soon as the new site is live requesting that they update the links to the new URLs as any type of redirected link will have less power than a direct one.
When you flick the switch and turn on the new website it would be a good idea to create a new XML sitemap and use Google Webmaster Tools to submit this to help Google more quickly discover your new pages. It would also be worth linking to this XML sitemap from your robots.txt file so that all other search engines also get this benefit.
There will inevitably be a period of instability after the launch of a new site. There is no easy way of determining how quickly search engines will update their index. You can benchmark how many pages they have indexed using the site: handler e.g. site:www.yoursiteurl.com will return the number of pages indexed in Google, Bing and Yahoo. Google Webmaster Tools is perhaps the best tool to monitor results as soon as possible, but your analytics package will obviously also tell you within 24 hours.
One last word of warning: double check that your developers do not upload a robots.txt file blocking any or all search engines from indexing your site - this happens all too frequently!