Ah - you won't get a duplicate content problem (as there is only one version of the url) - but it will take Google a few weeks to figure it all out (but you'll gain long term) - but you could use the redirection plugin to 301 redirect the old url's to the news ones.
That's what I would do - as you keep any page age and PR that the old urls had accrued, plus google will sort out the new indexing quicker