We already have a purchasing policy that covers most of the required elements of the prerequisite and the LEED-EBOM Policy Model, but not all of them. Do we have to write an entirely new policy?

You can submit an existing policy with an amendment or attachment that addresses the missing requirements, as long as all new sections are officially adopted along with your existing policy. You may want to consider highlighting the required elements that are covered in your existing policy so that the LEED review team can easily confirm that you’ve addressed everything.