I have uploaded a Downgrade program to convert the setup.fix file from Version 6.x back down to 5.13.
I have tested this on my local system and it appears to work so far.
This is a Windows executable and will not run in DOS.
As always, please make a backup of all your files before running this! This will downgrade the SETUP.FIX file as it is the only file that changed between versions. All other data files should be the same between the two versions.