Thanks for all information updat.
Finally I can update a baremetal HP N40L (mod bios & using onboard NIC) with 6 HDDs, from nanoboot DSM 5.2-5644 to Jub1.03b loader for DS3615xs with DSM6.2.3-25426 directly.
Data migration is straight forward & perfectly smooth. Only needs to ensure C1 disabled in BIOS.