When a product is substituted, it must be possible to mark the substituted material as Use-Up, meaning that the old material inventory is consumed FIRST from inventory before the new material or material variant is consumed.

When using plan groups in formulas, it is possible to define the sequence of consumption, however in this case, the old material that is no longer supposed to be purchased or produced must be on priority 1 to be consumed first from inventory - but it is not supposed to be purchased.

One way to solve this would be to consider the lifecycle state for procurement or production. Another way would be to introduce more sophisticated replacement rules.

If the solution is based on planning groups, it would be great to see that concept also enabled in Discrete BOM's.