Should we implement an advanced planning system before or after an ERP migration?

In most cases, before, or in parallel. The two projects address different layers: the ERP is your system of record, an advanced planning and scheduling (APS) layer is your system of decision. Because a planning layer consumes a small set of standard tables rather than replacing transactional processes, it can run on your current ERP and be re-pointed at the new one at go-live. Waiting only makes sense if your current data is so unreliable that no planning logic could use it, which is rare, or if the migration is weeks rather than quarters away. Given that ERP timelines routinely move, treating the migration as a hard prerequisite usually means postponing the planning decision by years rather than months.