If I did a paid upgrade on one leg of the journey, can I still place a bid for a different leg?

I booked a super-cheap (K) TATL trip, and upgraded the first TATL leg to biz (P) paid. I would like to upgrade the return TATL portion to PE, but at the moment there is no N availability, I assume that was the reason why I was not offered a paid upgrade. Now I still see the upgrade button on the itinerary website, but it leads to an error website, and I do not see the MyOffer button anymore.