I'm not an expert on this, but read some really interesting stuff talking about this on Matt Cutts blog were I seem to recall he covered this very topic
http://www.mattcutts.com/blog/
If you don't know he's one of the senior guys at Google, in fact so senior I can't see an "About" page on his blog - he might be a tad arrogant - or maybe I missed it.
Good Luck with the change - they're always a bit nerve racking :-)