setho212, your best bet is to OLCI (check in for your flight on-line) which you can do 24 hrs prior to your flight on Icelandair.com
I hope you, and everyone else who flies Icelandair, understand that when they make their reservation and select a seat, it is not a seat assignment...it is only a seat request.
Thinks can change. Usually, you get what is requested...but, sometimes not.
You never know for sure until check-in, which it is why it is important to check-in via Icelandair.com 24 hrs prior to your flight. Then you can actually get a seat assignment.
In your case, do an OLCI via the Icelandair website, and select your seat from what is available. When you select a seat with OLCI, it will be an assigned seat.
I hope this explains it, but the bottom line with Icelandair is to do OLCI at the 24 hr prior to departure time, to change to the best seat.
No need to wait in line in BOS at ticket counter, with OLCI your seat is set.