I called in this morning to inquire about changing both of my reservations - called DL in the US and KLM in NL - neither of them could make a date change to the outbound without repricing at current fares. Got someone on the line within 1 minute at both contact centers and both were very pleasant.
As mentioned above I did cancel within 24h with KLM but no refund yet - I asked them now to halt the cancellation and put a note on the PNR - since these flights are many many months away it's likely there will be a major schedule change by then anyway allowing me more flexibility to change one or both of the bookings.