I started looking around to see what I could do. It looked pretty much like I was going to end up paying a $400 change fee. But at Lisa's suggestion, I called IcelandAir. While waiting (it took quite a while to get through), I saw a glimmer of hope: flights booked through their web site from US origins more than 7 days before departure can, according to their conditions, be canceled within 24 hours without penalty. I couldn't figure exactly when I booked the outbound, but the date on the confirmation was one day earlier.
Eventually, I got through to an agent, who looked it up and found that I had made it with less than 20 minutes to spare. If there had been many more people waiting (the queue was ten deep when I first called), I wouldn't have made it.
Rather than cancel the ticket and make us start over, the agent simply changed it to the previous day so that we're actually flying on the days we needed to travel. There was no change fee or penalty. Everything worked out. Whew!
That's now two airlines on which we are traveling who have had excellent customer service agents who worked to make our trip to and from Finland work.