The only reason you would migrate from Accurev to ClearCase would be if:
- ClearCase was part of a IBM deal including much more than a simple VCS (usually many servers and other softwares)
- IBM support was part of the consideration (IBM offering a very complete and detailed support for all its software)
But beside that external context, I don't know of any migration based on the value of ClearCase alone:
ClearCase is only maintained at this point, with fixes and patches, but without any major evolutions (beside the CCRC client) since 2004. Accurev is still very active.
A "ClearCase 2" already exists and is called Rational Jazz SCM, part of the commercial offer called RTC (Rational Team Concert)
If anything, a migration from any tool to RTC would make much more sense.
Note that moving to RTC would be a very different move that just changing an SCM.
RTC provides a fully integration development lifecycle experience, with requirements up to builds and test plans, including SCM, instant messaging, full integration with Eclipse or Visual Studio (2005 or more), and so on.
That is what they call "Collaborative Lifecycle Management project, which coordinates the software development activities across requirements, development, build, and test".
The Jazz platform in itself doesn't include an SCM, but rather is an "open platform for lifecycle tool integration", i.e. an open-sourced Application Hub, in order to make any tool (Rational or not) able to communicate one with another.
Choosing the "Source Control component of Rational Team Concert" is usually done in the context of a much larger migration, aiming to regroup in one referential (the Jazz Team RTC Server) all development-related elements (sources, but also requirements, builds, tests, ...)