Agreeing that this was a nice gesture it do raise a few questions. I assume the purser did this on his own initiative. If used the right way (at the gate) it could be a great incentive of travelling more with SK as a EBG. If done wrong and in front of other passengers in-flight, it will send a strange message.
I hardly think we will ever see an upgrade policy on SAS (or any europeen carrier for that matter) due to the flexible cabin system. The situation here in Europe is not in any way like the F class issue US carriers have with seats that are fixed to a cabin.
But again, if this has something to do with the (IMO) stupid seating policy it could mean that there are so many complaints about it that SK staff feels required to do something about it.