The easiest and quickest way is to simply perform an OPTION 21 full save and then relpace the disk drives, configure RAID or Mirroring and add them into the system ASP and then IPL from D mode, which is your tape drive that you performed the backup on which must be your alternate IPL device and then load back everything from tape including the LIC, i5OS, IBM licensed programs and your user libraries, DLO and IFS. On a machine with 6 disk drives, that shouldn’t take more then 8 hours to accomplish given the speed of the tape drive. The backup and restore instructions are in the Backup and Recovery Guide for your version of i5OS.
The other option is to replace a few disk drives at a time depending on available disk space because you need to first remove the drives which means the system has to move the data onto the remaining disks. Then replace a few at a time. This is the long method but without a complete reload the only other way.