Short answer: You do not write a new plan. You add three pages to the existing one: the list of assumptions the plan rests on, the scenario in which each assumption breaks, and the decision that will be made if it does. A good strategic plan stays as it is; what changes is that management knows in advance when it stops being right.
The longer view
Every strategic plan rests on hidden assumptions about the world: that trade with a given market stays open, that energy prices stay within a certain range, that regulation does not change. The work is to bring these assumptions out of hiding and put them in writing. Once written, they can be checked every quarter, and the plan turns from a document into a navigation instrument.
The integration requires one working day of the executive team with someone who reads the geopolitical layer, followed by a one-hour quarterly review. That is the difference between a plan that goes stale the day the world changes and a plan that knows how to adapt.
This is a reading through the seven layers of The Blue Pumpkin, the method of Dr. Bella Barda-Bareket.
What to do tomorrow morning
Open the existing plan and write next to every major objective the external assumption it rests on. That list is the first step toward everything described on this page.