When you uninstall Workflow to downgrade, it's also a good idea to remove any remnants of our GACced DLLs from the GAC as well (if they exist). The GAC is shown when you navigate to C:\Windows\assembly in Windows Explorer. The DLLs to remove are as follows:
LogicBase.Core (and associated resources)
LogicBase.Framework (and associated resources)
LogicBase.Framework.DatabaseDrivers
Symantec.SymQ (and associated resources)
Symantec.Processmanager.WebServiceProxies
If these DLLs still exist at C:\Windows\Assembly after uninstall, stripe them, and then right click and select Uninstall.
If you've uninstalled and you can't remove them because they're "in use", you can force them to be uninstalled by deleting the appropriate folders in DOS:
C:\Windows\assembly\GAC_MSIL\LogicBase.Core
C:\Windows\assembly\GAC_MSIL\LogicBase.Core.resources
C:\Windows\assembly\GAC_MSIL\LogicBase.Framework
C:\Windows\assembly\GAC_MSIL\LogicBase.Framework.resources
C:\Windows\assembly\GAC_MSIL\LogicBase.Framework.DatabaseDrivers
C:\Windows\assembly\GAC_MSIL\Symantec.SymQ
C:\Windows\assembly\GAC_MSIL\Symantec.SymQ.resources
C:\Windows\assembly\GAC_MSIL\Symantec.ProcessManager.WebServiceProxies
After you've uninstalled from the GAC (or deleting the folders above, if you had to), delete EVERYTHING that exists at the original install location. Also remove your ProcessManager database (if you're downgrading). Then try running the installer again.