If you’ve changed the slug of an existing post type you can just change it back and all your content will still be there. A tip for the future is that you can, instead of changing the slug, change the rewrite slug. That’ll effectively make it appear as if the slug has been changed while you still maintain all your old posts etc.
What is your current post type now then? Your description is a bit confusing 🙂
Also, this isn’t really an ACF issue so I think you should post in the support forum for Formidable Pro as well 🙂
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