Ages ago I did try installing 1.6 versions onto virtual machines - they got a bunch of errors, but they still *sort of* worked.
You might be able to acheive what you are after by doing the following for both CTMS and CTS-Man:
Backup your 1.6 installation
install 1.6 onto a VM, restore the configuration from the production system
upgrade to 1.8 (or later)
Backup the configuration
Install a new version of 1.8 (or later) onto the VM
Restore the backed up configuration to the new VM installation