<p>I work in an multi-site environment that's currently using Rational ClearCase for source control and Rational ClearQuest for issue tracking (I accept your condolences ahead of time).  As a rough estimate I would say this is supporting 200 engineers.</p>

<p>How would you effectively migrate this SCM methodology to a comparative, all Open Source tool suite?  Not only would this save literally hundreds of thousands of dollars but I also believe it would improve developer productivity and very little downtime compared to the current system.</p>

<p>Platforms in use include Windows, Linux, UNIX and Solaris.</p>

Clearcase is awesome.  I use to think like you but then after moving to perforce I realized how great dynamic views are.
I actually asked about this in
another question
.  Basically it is really, really hard and is made much easier if you can live without your revision history.
As for bug tracking my experiences are that open source bug tracking tools are terrible.  However using triggers it is usually very easy to integrate them with open source source control. As an example here is how to
integrate bugzilla and subversion