Is it possible to use Substitute half way through Production Run?

Summary:

One of the WO component is short supply. Hence we do not have enough quantity available for the whole Production Run.

Is it possible to use up the primary component on the production run (work order) and then use the substitute component on the same work order?

I could only swap the substitute component before WO is started, however once we have transacted system does not allow to swap to substitute components.

Is there any setup being missed? Isn't that the whole purpose of defining substitutes on the Item Structures/ Work Definitions?

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