In general, if you're trying to follow the recipe as close as possible and need to substitute hops, yes you want to tweak the amount based on the AA% of the hops you are substituting and the AA% called for in the recipe. Which would accomplish your goal of hitting the IBU number.
However, there's nothing that says you can't change the recipe a little by substituting a different hop and not changing the amount. It's all up to you.
Edit: To clarify, you want the AAU's to be the same. AAU = AA% * oz. So if the recipe calls for 1 oz of 6% cascade, that's 6 AAU. If you want to substitute 12% columbus instead, the math is easy, you'd have 6 AAU / 12% = 0.5 oz. If you're substituting 9% centennial, you'd use 0.67 oz.