Hmm, I was under the impression that ARN-AMS flight arrived before AMS-SLC, just that it was too tight to make. Thus waiting until the next day would require a stopover. If the AMS departure was moved up to before the arrival from ARN, and it was less than 24 hours, it shouldn't have been an issue.
And as mentioned, the fare class could have been forced.
The best thing to do at the time would have been to not accept the change, say you wanted to think about it, and then come here for advice (and/or talk to Seaborne). Oh well, hindsight. Maybe you can get DL to budge, and/or maybe the lower fare class will open back up (ExpertFlyer is good to monitor this).