I created a simple database with related PRODUCT, ORDER and ORDER_LINEITEM tables for a nutritionist. Some of the PRODUCTS are the same but contain twice as many capsules. Is it possible for the ORDER_LINEITEM to change automatically from one PRODUCT to another PRODUCT if more or less capsules are required for that whole ORDER.

You contradict yourself, Stefan. You say that the "PRODUCTS are the same but contain twice as many capsules" and then say, "change automatically from one PRODUCT to another PRODUCT."

Seems to me, that a product, such as a bottle of Vitamin C pills, would need a unit count. So, you'd have 1 bottle, 100 count. That wouldn't have the same SKU as a bottle of 50 count, would it? It's a different item.

For reporting purposes, they may share an ID that will relate them back to the same parent product family. So, you could report on all purchases of Vitamin C.