URL changes when moving a forum

[gecko1]

I am considering moving my phpbb forum to the latest version of vbulletin. While they have a script to do this, my URLs are all going to be different.

The phpbb forum has the mod to make the URLs SEO friendly.

I imagine I need to set up some mod rewrite rules to make sure the existing URLs work.

What's going to happen if I don't do anything with this? I imagine I will lose some search traffic. Regular users probably won't notice anything, other than the change in software.

Is Google going to eventually recognize the URL changes and update its index?

The site currently does fairly well in Google and could be considered an authority site. Any tips or advice?

[BradleyT]

IMO you really need to 301 all old URLs to their new URL.

OR

Look at which URLs have inbound links and 301 those so you pass on existing link juice.

AND/OR

Look at traffic and which URLs have the most traffic and 301 those.

#1 is probably a ton of work. A combo of #2 and #3 is probably more realistic.

Also take the old URLs out of your sitemap XML file and add the new URLs. And keep a close watch on what Google Webmaster Tools is reporting.

[Status_203]

If you don't 301 all urls then watch your logs for 404s to old urls and add additional redirects as they happen.