Every model/animation file has a version number encoded in one of its first bytes. It's there to avoid compatibility issues. They increased the version number in SH (and in BT as well, I believe). To be honest, I can't see any reason why they would want to do that, unless they just wanted to prevent peeps from using the expansion pack models in vanilla AA, or unless they implemented some new features (which, as we all know, didn't happen; it just seems that the expansion pack models are rotated).
Cobra at MRU has once offered to convert models from SH/BT to AA for anyone interested.
Really? Well, I'll ask him about that, then! It's nice to know why simply converting them won't work, now. BT models can easily be thrown into SH without any problems.