Hmm it has always been the case that if you booked a fare, requested an upgrade, confirmed it, and then downgraded you would have to pay the fare difference. While the policy has not always been equitably enforced it has always been in place, but the only difference now is that it's actually much easier to enforce (though frankly was not that difficult before).

Unfortunately (and this is especially true on long-haul flights) it makes sense that the airline would want you to pay for economy space as many international flights have relatively limited space and based on standard airline pricing model flights get more expensive within 28, 14, 7 and lower days with the highest pricing typically occurring 1-2 days prior to departure to capitalize on business or pleasure travellers who simply need to be on that flight.

I frankly don't have a problem with this policy and it isn't something that can be easily blamed on eupgrades.