First you have to make sure they are being used in any cluster configuration and that the disk group is not included in any /etc/vxfendg file on any system and the disk are not in any /etc/vxfentab file on any system.

Then you need to import the old coordinator disk group onto one of the servers that has access to it and destroy the diskgroup. After that you will be able initialise the disk in the normal fashion and add them to new or existing disk group.

vxdg import diskgroup
vxdg -o coordinator destroy diskgroup