Hi,
I've tried searching for this but can't find the answer and it's really confusing me!

If I book a WTP flight Dub - Lhr - (anywhere in US) return for cash and want to upgrade the transatlantic segment with avios, will the cost to upgrade be the same as if I had just started the journey from LHR or will it be cheaper because I started in Dub?
I've tried doing a dummy booking but it upgrades the whole itinerary.

Thanks