I moved a site from Wordpress to Umbraco and had some indexing issues. You have to stick with the basics like 301 redirects as well as .xml file re-uploads. What you will want to do is look at your webmaster tools about 1 week after launch and see whether or not everything worked.. . it just happens sometimes that it doesn't. As well, from the studies I've read, there is sometimes a percentage of decrease in traffic after a migration. I don't want to arbitrarily throw out a percentage because it may have changed. I would look that up and let the client(s)/stakeholders know. You may be able to combat that with a short-run PPC campaign to balance that percentage out.
