I am due to start a DONE4 next Friday, however something has come up and I would like to change all flights excpet the first ARN-LHR. This would make me stopover in London instead of transit, will this make a difference.

I expect there is a penalty for this but cannot find how much this is? As I am changing the first longhaul flight would this cause the ticket to be repriced?

Thanks for any help.

Route change is $125. They will also recalculate your taxes.

However since you change the first long haul flight the ticket will probably need to be reissued at the current prices. There is a 10% discount on, so that could be good.

As you are not changing your first flight, no need to reprice your ticket.
You say you want to change the rest of your itinerary after LHR, your LHR stop becomes a stop over instead of a transfer. If this involves additional taxes and i suppose it does, you would have to pay those. Plus usd 125 change fee as your ticketed points may remain the same, but i think one pays the change fee for changing stop overs into transfers or vice versa. Additionally, airlines may change you a service fee, depending on the airline, for handling these changes.

Actually, the usd 125 fee is questionable: i got away with changing from stop overs to transfers a few times on a RTW ticket without paying the usd 125.
This was not on a OW round the world but on Star and involved Lufthansa both times. I think they wanted me away from their counter as quick as possible and conveniently forgot to charge me for extra taxes and a change fee No service fees either (LH airport desks in SIN and ATH).