I've played with a few sample online bookings at the Aeroplan site, and compared all-economy versus all-business class, same identical segments/itinerary, and there is no difference in the taxes/fees. However, I've seen people mention that a later recalculation of taxes could result in an increase or decrease, which I think I can understand, though I'm not sure where the source of the variance could be.

Anyways, I was also told by an Aeroplan phone agent that taxes will be become higher if I upgrade a from a Y to J segment on a business class award, just because the taxes are higher for business seats. Is this true? If so, why is it that the taxes look identical for a Y or a J itinerary, when I make the same booking online? Or is this the case only for specific partner airlines?