You have booked roundtrip fares which permit open jaw bookings to any destination in EU (Area 2). "Roundtrip" fares still involve a separate fare in each direction of travel (the roundtrip part just refers to the travel requirements for combining these cheaper fares together on one ticket). So you can change destination in EU without a re-fare on outbound fare as it will still meet fare rule requirements for that fare. Basically, just have to continue to meet min stay (typically Sat night/7 days) and max stay (12 month) requirements for the outbound roundtrip fare to keep it which won't be an issue. There is a change fee of EUR 150 on many cheaper coach EU origin fares (EUR 200 for Biz fars) -- since you booked a roundtrip, both outbond and return fares are EU origin fares (similarly, US origin roundtrip bookings book into US origin fare filings for both outbound and return fares which have different pricing/fare rules/penalties from EU origin fares). Change fee would be waived due to schedule change. Suspect it will be a challenge to simply refund return fare while still keeping outbound fare since you would no longer meet roundtrip booking requirement of outbound fare (not clear to me that EC261 rules would require this).