That is about right. It's just how the fare for your trip is constructed, it's made out of 2 components: outbound and inbound.
You could upgrade either (or both) but not segments within.

Having said that and by looking at the LIS-BNE fares, upgrading the outbound or the inbound should set you back about $2k-2.5k US, depending on how much you paid for your business class ticket.