Are you building the rod and plan on removing the length from the butt section? Although all taper designs are calculated via the full length of rod....arguably the tip down 90% is the most influential in the performance of the taper and the taper can sometimes be shortened, although it always changes the rods performance to some degree, butt end length is what can be removed. If, however, you are asking if you can just take 3"s off the tip....then no...this will at the very least change tip performance and casting efficiency significantly and likely the effect will be felt further down the taper as well.
Modifying any taper from it's original design on either end is always a crapshoot that almost always turns out to be less efficient than original design.....if an 8'9" configuration is important to you....sell the 9' rod and buy your preferred 8'9".