Dear sir,
There are no way to downgrade into 16.2 or to Altium. We have to using step by step as below to make the board in Allegro 16.2
1./ We have to use netlist 17.2 to import into board rev 16.2.
2./ Convert pcb footprint to 16.2
3./ Export placement, fanout, routing, plane, silkscreen....
4./ Double check, QA and make gerber files again.
Please trust and give us this project. We already installed aall these tools on my computer and we can make sure it.