Yes, it changes the URLs to more friendly one. So after Google SEO also we can redirect the URLs to more friendly one.. Can you tell me what will be the things which we are going to losse, also we have too many forums to make the scroll less we use forum tabs plugin in mybb, is there any such plugin available in xenforo also?
If your board is on the root of the domain, I don't think you require a redirection. I did the same for CODForums and it skyrocketed on switchover from vB4 to xenForo. Now I'm doing twice that growth a year ago. It's hovering between 1000, to 400 visits per day. 1000 is max, and 400 is the new average.
The only times that a redirect is necessary on switches is because:
a) Your filefolder is not on root; example here:
http://www.everythinghalo.com/forum (all threads are currently redirecting to root)
b) Your prior software required a plugin to have friendly URL's - in this instance, you're going to need to use htaccess, as xenForo does not have that plugin (the one you're using for MyBB) coded in or used as plugin. And even then, xenForo has SEO built into the whole software. "Broken pages" or URL's are handled by xenForo's embedded 404 pages, which is handled by xenforo natively - therefore, your broken pages will come back to your site. Nothing to worry about here.
c) You switched software - which results in URL structure changes, and indexing changes. This is a simple swap of filefolders, and redirection via htaccess. (from old threads to your new forum)
I honestly do not fear your site will regain traffic. In fact, it'll be indexed faster, and soon afterwards, you'll get more.