Hi Sandi,
Please use 301 redirects from migrating from old urls to new urls. As this will intimate the search engines that the old url is migrated to new url permanently. Earlier i have migrated my word press blog (http://ifsc-codes.in) to asp.net website to maintain dynamic content and huge data. At that time am not sure about this setting and i have almost nullified my Google crawl rate and no organic visitors. After that i have realized how much importent factor is this. So i did some coding techniques to address 301 redirection and right now it is getting better. Use your robots.txt file to redirect these urls. Note: Please use 302 if this migration is temporary. Try to create categories to retain old urls if possible. Hope this will use full.