Hi Ken,
SNP engine will create a PR for the product on which there is demand. So, rather than trying to manipulate SNP Heuristic to create an order for the old product , you may want to copy the forecast to the 'old' models. So if the forecasting happens at both new and old, you may copy the forecast to old and then release to SNP. And then, the PR will always get created for the old model. And this might help in interchangeability scenario too w/o the need of BADI.
Hope that helps.
Shweta Gupta