Alternate parts on sales order with assembly

Hello all,
we are creating a “master” bom with many alternative parts so that we choose the specific parts on a sales order depending on the need. If the sales order is just for a single component, it is easy to select the button and choose the alternative part that applies. However, if the sales order line is a top level assembly, I do not see how to be able to choose the specific alternative parts of the child components, as the box is greyed out. Not sure if anyone has any ideas?? Thanks all!