The slot numbering is fixed in the 4.x.x branch of 4U60 SEP firmware. They will however have an issue if they upgrade to that branch of firmware. The image you provided shows that they have Seagate, Fujitsu, etc. drives. They will lose access to those drives if they upgrade to the 4.x.x branch because firmware does not allow non HGST, WDC, SanDisk branded drives in the enclosure beyond 2.1.0 firmware.
Tags: drive, firmware, JBOD