Thank you everyone, in the end we used the new tickets. It was not an option to stay the night at ARN (traveling with a child and flights purchased as separate flights through Expedia, because SAS did not offer them as combination). TFS will have testing services starting January 4th, and TFN seems to have just stoped the service completely. A lot of bad luck with the New Year's weekend (all closed) and the airports not having ensure continuity for their labs. We will talk to SAS and Expedia but likely we just lost 800 Euros...