I just did that. I am currently Plat on NW but due to a change in assignment, I needed to switch to Star Alliance or OneWorld. I called AA up and told them about my plans. They said they wouldn't comp status but would sign me up for the Platinum challenge. Earn 10,000 Q points within 90 days and you get Platinum. The only downside is that AA Platinum is equivalent to NW Gold. You could sign up for the Gold challenge and get the equivalent of NW Silver relatively easy if you are going to do much travelling at all. All it took for me was one business class trip from YYZ to MEL and bingo, Platinum Status