Windows Server 2008 R2 - Coverting to retail license from volume?

Hi,

I'm in a bit of a odd situation where we have a 2008R2 server that is currently on a volume license.  This agreement is coming to an end and I want to move the server to a retail license with out the pain of rebuild.  I know this can be done in 2012 using slmgr but I can't find any examples of doing this with 2008.

If it can't be done, then I do have the opportunity to upgrade the server to 2012R2.  The question there is can I upgrade a volume licensed version of 2008R2 with a retail version of 2012R2?

I know this sounds like a disaster waiting to happen but at the moment I am trying to exhaust all the possibilities to try and find the path of least resistance to get onto a retail license.

Thanks

Rob