I work with a lot of organizations to refine or create their case for support. Recently, in two instances, the team came to the table wanting to turn their strategic plan into a fundraising document.
It doesn't work that way.
A case for support is an external communication tool. Its job is to articulate, in plain language, what problem your organization exists to solve, why you are the best suited solution to that problem, why it matters now, and what partnership with you makes possible. It's designed to move someone from interest to investment. It's not a brochure, a grant proposal, or an annual report. It's the foundational argument for why someone should fund your work.
A strategic plan is an internal operational document. It tells your team where you're going and how you'll get there. It's full of KPIs, timelines, and organizational priorities that matter deeply to staff and board but mean almost nothing to a prospective donor reading your materials for the first time.