Many people are considering their options when it comes to moving/migrating Enterprise Vault from their existing hardware to 64 bit - usually to Windows 2008 R2 x64, as a preparatory step of going to Enterprise Vault 10.x
There are several ways to get there.
I certainly wouldn't recommend any kind of Operating System in place upgrade. It just always seems to end in unhappiness.
I would suggest one of these ways:
Use the Enterprise Vault Server Settings Migration wizard. It is described in detail, and available for download:
I personally have not yet used that.
Use the Migrating EV 9 to 64 bit documentation available here:
I have used that a couple of times lately, and it's worked very well.