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.
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.
Platforms in use include Windows, Linux, UNIX and Solaris.
First, why do you think this would improve developer productivity? I haven't used ClearCase much, and ClearQuest not at all. What about these tools is hindering development?
Once you know what you want, you need to look at various tools. I'm fond of Subversion for SCM, as a general rule, but there are situations it isn't well suited for. I have no strong feelings on specific version tracking systems.
Bear in mind that migration is likely to be a really big project, depending on what you want to bring over from the Rational systems (checking everything out in ClearCase and starting entirely new projects in Subversion will be easy, but any history you want to keep is a lot more work), so there will be no immediate dollar savings. Moreover, switching tools is going to reduce developer productivity for a short time (possibly very short), so this is best seen as a long-term move. Make sure you get the tools you want up front, since you aren't going to want to do migrations very often.