Make VMWare independent Disk dependent again

Make VMWare independent Disk dependent again

I have just converted an old VMWare image that was created in VMWare
Workstation 4 to the VMWare Workation 6.5 format.
The image has 3 virtual IDE disks that are set to "Independent". Since I
am working with snapshots I would like to make the disks "Independent"
again so that their state is saved / restored when I set / restore
snapshots.
However, if I go to "HardDisk Settings -> Advanced" the "Independent"
checkbox is greyed and I can not remove the check mark.
Do I have to convert the disks to a new format somehow? I did check the
vmware-vdiskmanager but could not find any apparent option.
Any idea?