Managed to pay for an upgrade on the outbound leg, but the return leg is showing that I "cannot upgrade this leg". Could anyone kindly shed some light as to why this might be? Is it a matter of trying to upgrade the return leg on the day of the flight or is there some way it can be done at the airport?
Is there actually any space in Club? (The quickest way of checking that is to see whether you can book a new ticket on that flight.) If Club has zero inventory, then you will not be able to upgrade. If there is inventory, then a fare rules upgrade ought to be possible, but you may have to call to do this if the website isn't allowing it online (which could be because you have already upgraded the outbound flight).
If you knew that you needed Club in both directions, wouldn't it have been easier just to buy Club in both directions to begin with?