Rebooking/Rerouting
Prior to departure
Origin TC2/3
(1) After ticket issuance - date/time/oneworld carrier/inventory changes are permitted provided ticketed points remain the same.
(2) Changes other than to Date/time/oneworld carrier/inventory permitted at a charge of USD125.00 per transaction
Europe is TC2.
IIRC nothing prevents you from changing your first segments, but this leads to a reissue of your ticket (per rule 2 above) and you have to pay the USD 125 fee. A reissue also leads to the application of the current set of rules and in your case this would mean that an e-ticket would need to be issued and the 16-segment-per-itinerary limit would apply. You'd need to reduce your total segments to 16. On top, if the ex-ARN price of a DONE5 has increased in the meantime, they will charge you the new price (the whole itinerary is repriced IIRC).
The experts will be able to confirm if this is correct.
