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I have never worked with Actinic, but any web-application would have a HTTP server serving content. If your domain remains the same but your new site's URL structure changes, then I would look at rewrite rules for pages/categories redirecting to the new content. In essence you have an old application which you will turn off (it will not serve any data) as it will be replaced with something new. So on your new system (I sounds like it will be IIS) you will configure a series of rewrite rules  - any indexed traffic or linked pages would then redirect. GWMT will be a good guideline (as well as your server logs) for 404's. If you can't fix them pro-actively, looking at the logs will be your best option. If you don't rewrite you will pretty much drop pagerank and SERPs.