the short answer
Hand off clients as a warm, endorsed upgrade, never a silent reassignment. Move newer, lower-touch, well-matched clients first and keep your deepest relationships until last. Introduce the new coach yourself as someone you hand-picked and trained, who will be more available day to day while you still oversee the plan. Overlap for a few weeks so nothing drops, then set the new normal clearly, and stay lightly visible so the client feels promoted, not offloaded. Done this way, very few leave.