I think the first answer you received is good if you can take the risk. If you want a fare difference upgrade on a single segment, you can call and ask about changing the dub-lhr segment only into business but it will count as a change and the whole fare will be recalculated and this could be expensive. It is even possible that the version of upgrading the whole outbound to j-i was cheaper than upgrading dub-lhr only which may be why the system offered it.