The flash image should never be altered. The flash is signed with a hashing algorithm to provide security and prevent tampering. You can check question 2.23 on the I210 FAQ:
The NVM firmware images for the I210 are signed. Once the I210 successfully initializes with a signed firmware image, security is enforced (unless security is disabled by pulling down pin 12) and the image cannot be overwritten except a by a new signed image that has the same or higher security revision
Not all fields in the image are protected. Fields that are normally subject to change by the OEM, such as the MAC address, PCI subsystem ID fields and LED customizations, can be changed without invalidating the digital signature.
There are two ways to change the MAC address:
1) Using the LANConf tool or the EEUPDATE tool (recommended method):
To change the MAC address you can go to the Registers section the LANConf tool menu and manually change the MAC address of your devices as you can check on section 2.5 of the INTEL® LANConf User Manual (provided within the Intel® Network Connections Tools):
The Registers (MAC and PHY) menu allows the user to read or write to the MAC and PHY registers.
The EEUPDATE tool also can change the MAC address of a device as you can check on the MAC ADDRESS FILE FORMAT section of the eeupdate.txt file that is provided with the Intel® Network Connections Tools
2) Programming the MAC address registers directly using the host interface (only if you don’t have the NetworkProgramming Tools):
You can write directly to the MAC address registesr 0x00-0x02 using the PCIe interface, you can check this thread for more details: i210: absolutely necessary words in iNVM