From what I understand...
Weight x %AA = Weight x %AA
In your case that would be:
Weight X 6.5%AA = Weight x 10.4%AA
This essentially boils down to 6.5/10.4=0.625 or 5/8 of the called for weight.
If the recipe calls for 1oz of 6.5%AA hop, the you would use 5/8oz of 10.4%% hops.
Here is a IBU calculator that my be useful if you want to play around with the hop weights, %AA, and addition time.