Ok here goes nothing...
First you need to block the NIU port (HDU port)
BLEQI:EQU=1-0-60-0;
Then unmount the volumes (of the working HDU)
FIMOE:SUBFS=SYSSUBFS11;
FIMOE:SUBFS=SYSSUBFS21;
Unmount the disk/node itself
IOSIE:NODE=SYSN,IODEV=SYSDISK1;
Turn off the HDU and replace with a new one...
Turn on the new HDU, then mount the disk with the following command
IOSII:NODE=SYSN,IODEV=SYSDISK1,SUBFS=SYSSUBFS11&SYSSUBFS21,SIPOS=3;
Format the disk for the following volumes and sizes (50 is a very big size)
FICRI:SUBFS=SYSSUBFS11,SIZE=50;
FICRI:SUBFS=SYSSUBFS21,SIZE=50;
Mount the files on the recently formated subfiles
FIMOI:SUBFS=SYSSUBFS11;
FIMOI:SUBFS=SYSSUBFS21;
Unblock the NIU port (HDU port)
BLEQE:EQU=1-0-60-0,ALLFM=YES;
This should be it. I've done it a few times, you shouldn't have any problems.
Let me know if you have any problems
BR
Issiro