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.