Don't think they authenticate based on MAC address because I had my HG8240 successfully configured to replace HG850a.

Try to open up the saved configuration with notepad/browser and look for the field as below:

<PassWord PassWord="0000123456"/>

PassWord is the optical authentication between Huawei ONU & OLT (not too sure about other brand though).

What you can do is, reset the new HG850a to factory settings, and insert the PassWord through web interface by using "telecomadmin" username then save and reboot the ONU.

If you stay in PJ area, I wouldn't mind lending you the HG8240 for you to test it out...