Note that it is NOT strictly an upgrade voucher.

What happens is that you can book in CW for the Avios required for WTP, or WTP for the miles required for WT. (You cannot book F for the miles required for J).

There does not, therefore, need to be availability in both classes, which would be the case if it was technically an upgrade.

Note that the 2 upgrades need to be used in the same booking. You can upgrade you and a colleague one way, or yourself in both directions.

You CANNOT upgrade yourself one way and then use the voucher again on another occasion to upgrade yourself again one way.

The maximum 'value' of the voucher is therefore 25,000 Avios on most routes - ie those routes that are 100k in J return. The absolute maximum value would be 50,000 Avios in the unlikely event you could get J to Australia!