Too early to say for sure, and it'll still depend on prices of the new unsold 24-70's on the market. If that goes down, the prices of the used ones won't go up. What will it be in a few years? It'll be a definite downtrend years down the line (matched for inflation), but for the current time, i don't think there will be marked changes in the price of whats being sold on the used market...
I asked a friend if he thought they'd still sell the Mark I. I kinda figured they might, but he thought no. They upgraded for a reason and probably don't have a need to sell both.
If its any indication, the price of the 70-200mm f/2.8L IS went up after the new MK II was introduced. The price of used supertelephotos has also jumped after the new much more expensive lenses were announced.
Expect prices to rise, at least for the next several months.