Thanks for the pointers, very helpful.

I booked D on the return legs, and there's three of us travelling together, so perhaps there's only J left on those flights?

I can't get the fare rules from my itinerary page on VN, it just says "There was a problem retrieving fare rules for your itinerary. Please call the airline".

When booking, I checked the fare rules on ITA matrix, where it just said "Changes permitted" but didn't have any stated fees. I think it did list some fees if changing rail segments in the UK in the 7 days before departure.

Might try calling VN today based on the itinerary message, and failing that, call Expedia back and ask another agent.