Peter,
Officially Windows 8.1 is NOT supported
Symantec Encryption Desktop 10.3.1 for Windows Release Notes (page 1)
http://www.symantec.com/docs/DOC6697
■ Compatibility with Microsoft Windows 8
This release supports the installation of Symantec Encryption Desktop on systems running Microsoft Windows 8 Pro and Microsoft Windows 8 Enterprise, both 32- and 64-bit versions
but this doesn't mean that you can try to install SED 10.3.1 on Windows 8.1. Most probably you will not have to unencrypt drive like it was with the previous version.