Yeah this is old hat, I know.  The Preview 5 release was news about three weeks ago.  There wasn't a need for me to upgrade until today.  Both Derik and Casey did great write-ups on some of the issues in migrating from Preview 4 to Preview 5 (found here and here respectively).  I've been using their posts to assist in the transition.  However, as I was making the move I kept getting the following error:

Could not load file or assembly 'System.Web.Routing, Version=0.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

I was following the advice that both Casey and Derik laid out for upgrading the web.config of all old System.Web.Routing references and yet I was still getting the error.  I could search the whole solution for "0.0.0.0" and not a single result would be found.

Turns out that in my haste I forgot to upgrade the MvcContrib library that we are also using.  It had a reference to System.Web.Routing (0.0.0.0).  So it was a little oversight on my part and after I upgraded to the latest MvcContrib, I was back on my way to upgrading, routing problem solved.

Cheers!