I'm with Hartley on this one. But it all depends on your content.

If you are going to have (and maintain duplicate content on all of these URL's) the the re-directs from the old URL's to the new URL's only take care of half of your problem, you still will be cannibalizing the rankings of these pages as they will be ''dinged' for duplicate content (worst case scenario) or at the very least, search engines will not know which version is to be representative, will not be able to discern any difference, and instead of trying to figure out which version should rank higher, both will receive a lower relevancy score. This is where the rel=canonical tag comes into play - which essentially tells Google, "hey, this is the representative version of this content that I want indexed," and to implement this you would need to add the <link rel="canonical" href="final destination URL goes here" /> to every URL that has duplicate content, all pointing back to the "champion" version.

Whenever you change URL's, that you wish to remain indexed you must do a 301 redirect for continuity of both page-level authority and so users/crawlers can make it to the correct final destination.