Here is the LEGO "LEGO® Power Functions Extension Wire" (8886):
We can see that the left connector can connect to old 9V electric bricks or wires and the right connector can be plugged to the new Power Functions connectors.
The LEGO website gives this description:
Power up your LEGO® Power Functions creations with this 8" extension wire. Make your models bigger, better, and more mechanized than ever before!
- Allows you to connect your LEGO 9V electric systems to the new Power Functions!
- Extension Wire is 8" (20cm) long
Does this cable work in both directions?
- Can it power a new Power Functions motor from an old battery box?
- Can it power an old motor from a new Power Functions battery box?