I think that using a different material can definitely cause a change in the product classification of a product. The overall impact of the material being used depends on what the material being used is and whether or not it has been safely used in similar applications in the past. Introducing a new material for a device that interacts with the biological system that hasn’t been used or proven safe in the application that the product is intended to be used in poses a risk that needs to be addressed and would likely make the device a higher class device to ensure that the risk associated with the new material is sufficiently tested.
In the case of the syringe, I think the decision to classify the device would come down to the fact that Plastic B has never been used as the material for a syringe, which would make it a higher risk device and possibly a class III device.