As you probably know, Actionscript 2 and Actionscript 3 projects aren't completely compatable - they can't be, what with all the changes under the hood. Migrating your own coding techniques is hard enough, but migrating projects can be so much easier and nigh-on-impossible. So, how could you make a mish-mash of Actionscript 2 and Actionscript 3 movies play nice?
Well, there's a few solutions, and several classes that claim to do the job, but the best I've found is FlashInterface. It might take a bit of figuring out at first, but the Events based model means that I find it quite easy to use.