Protect it before the resignation. Hold at least two named relationships inside every significant distributor, keep a company-owned relationship record with contacts, commitments, dated history, and next steps, and maintain a one-page handoff document for every top-tier account answering who the people are, the state of the relationship, what has been promised, and what would break first without attention. Companies that do this see top-account revenue hold flat through a transition year, and sometimes grow.