The award booking engine, when changing flights, tends to have issues showing the correct mileage amounts. I believe it ends up showing the correct amount when you get to the end and have the chance to pay or not make the change.

For example, I have a standard Y award to Europe on one itin I'm constantly checking for saver C (or even saver Y at this point). If I go in to change the flights, at least half the time .bomb claims the exact same flights I already have booked will cost an additional 35k miles.

Now, if I select those flights, the correct amount (0 additional miles) is displayed. Heck, right now, the return flights (currently booked in saver F) show as an additional 35k RDM for saver Y and an additional 85k RDM for saver F (which is what I'm booked in).

The long and short of it is that the website blows.