Agreed, it's fare difference fees I meant. But in terms of working it out, I wasn't sure if the cost would be:

1) The difference between the basic price I originally paid and the plus price now (utterly prohibitive, nearly 5x what I paid in the sale!)
2) The original difference between basic and plus when I booked (about cost neutral if we all want a bag)
3) The difference between basic and plus according to what's on sale today (which would be a bargain as it's higher buckets only and the gap has narrowed a lot)

Am guessing 1, but thought that I'd get an idea online first to decide whether to change or just buy more bags. Still, I got through immediately on the silver line last night, for the first time in months...