I don't see them downgrading the tickets to economy, that would be ridiculous, make quite a few customers (especially the ones willing to pay extra!) angry and lead to a really bad PR disaster.

My guess is that these customers will be seated in business but the ticket may remain Y+ in terms of benefits - and mileage credit - it offers. Service on board will likely remain the same anyway, given the similarity between SAS Plus crap and KLM wanderboxes.

Alternatively, I can see them keeping SAS Plus on board until the legacy bookings run out. But I consider this less likely.