Barry,
Backward migration script should be written in the “new” library. This migration process starts, when you save a plan to ARCHICAD n-1 version. Then AC iterates on all placed libpart instances and runs the backward migration scripts and changes the data stored in the project file.
It can’t be done with a simple library change, as it is done in the forward migration.
Gergely Fehér
Team Leader, Library Team
GRAPHISOFT SE