One alternative is to rename your first material, and assign the first material name to the second material you want to use.

However, if you're changing the materials to animate a change between one and another, I'm wondering if a Python script might do the trick for you (that is, if you have Pro Pack, P5, or P6).  That seems like the most logical and automatic way to accomplish it. I'm not a Python expert, but there are a few that hang around these parts. 8-)