Re: How important are SEF url's?
Keyword in the URL is great, but not essential (HTML title and link data far outweigh it). Not having dynamic URLs is helpful (static, or apparently static sites tend to get crawled more frequently and deeper - but then microsoft.com does OK. But it IS microsoft.com...)
If you can 301 the old URLs to the new ones, I'd do it - you'll retain the old link juice.
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