Upgrading from Server 2008 x86 to Server 2012 is not possible.
According the supported upgrade paths document for Server 2008 R2 (which is the most recent doc to mention Server 2008), "Cross-architecture in-place upgrades (for example, x86 to x64) are not supported." According to the supported upgrade paths document for Server 2012 R2, "Cross-architecture in-place upgrades (for example, x86 to x64) are not supported."
I believe the problem is that "Cross-architecture in-place upgrades (for example, x86 to x64) are not supported." You'll have to install Server 2012 as a new OS, which is probably what you should be doing anyway.
If you're talking about leaving the old servers in place and "controlling" them remotely from the new server (which you seem to ask about, despite the fact that this is contradictory to the statement you made about upgrading), that's probably possible, but a horrible idea. Just reimage the servers.