1) From any version of Picotel e510 original firmware you should flash to e510_26A_1015
2) From e510_26A_1015 -> e510_26Ato26B
3) from e510_26Ato26B -> e510_26B_501 

Known bugs:
 - Green Ethernet port LED becomes SYS LED.
 - sometimes ONU shows wrong DDM information(RX/TX/Voltage)