Thanks sbv3000. I'm not sure steps 9) and 11) as you stated on the above. I do know that there is 4.3 version of xpenlogy. Are you suggest me to try to upgrade the existing system to 4.3 first? Then, reboot the with XPE 5.X. If it succcess, uses synology assistant utility to search for the new system. But is there an option of 'Migrate' in this utility? Would the 'Migrate' utility safely upgrade to 5.X?
Besides, as there is huge amount of data in existing system and I would not want to buy an extra harddisk to backup. Safely upgrading is very important to me.