Originally Posted by
bioblot
They really should be more flexible with upgrades, basically allowing anything for the right price provided there is space.
They do, you can always pay the fare difference and change fee to upgrade to any class, even from discount economy to first. However, you may wish to sit down before asking for the price.
A POUG is (usually) heavily discounted. If you want to upgrade further using avios, then "the right price" to allow this to happen is a fare difference.