I don't believe there is any way to have a variable AA in FIRECalc. As an approximation you could split the difference. If you are now 50 (would have 50% bonds) and are running 40 years to 90 (90%) bonds you could average them at 70% bonds.
Also, on the Investigate tab you can look at "changing asset allocation" to get a feeling of the impacts, but that is not changing over time.