I've done it!
Just in case anyone else has this problem, this was the workaround I used.
I redownloaded the ODE as tomsem suggested, putting this in the documents folder. I'd checked my PW3 and this is where the ODE seemed to be, so I put it in the same place on the PW2.
This didn't work. So I wondered if totally deleting the New Oxford American Dictionary might force it to use the ODE. It took a while to find the NOAD, then remove it from the device.
When I did this, I selected a random word in a book. It used the ODE!!!
Thank you everyone for helping me to sort this out. I hope my solution helps anyone who has this problem in future.