Originally Posted by
Sabe59
I want an upgrade from W to J. I’d pay full cash fare for W. Would it be better just to book W and see if I can get a promotional cash upgrade to J?
You can't upgrade a cash fare with the voucher, only an Avios booking.
Remember this is not really an upgrade voucher.
It is a "Make an Avios booking and pay the Avios for one class down" voucher.