It's possibly possible. But as BA Loyal said, possibly not worth it.

If you a through fare DUB-SYD, then (assuming the fare is changeable) upgrading one portion of the ticket would normally mean you would have to upgrade the whole lot, ie DUB-LON-SYD. Upgrading just one sector would, in effect, be a reroute as it would involve splitting the fare and stuff. As you could probably guess, upgrading the entire DUB-SYD is gonna cost a lot of $$$ and would see you travelling separately from your client.