While loading, a plug-in's references are not in place during execution of its ReferenceMaker::Load() method - not until 3ds Max loads the entire file's contents. A reference maker can remap an old reference structure to a new one by providing a custom implementation of the function ReferenceMaker::RemapRefOnLoad()
To support the save to previous feature of 3ds Max there are some improtant recommendations to follow when making changes to the reference structure.