How to Remove a Kubernetes (GKE) Cluster node-pool without down-time
If you cordon and drain the nodepool before deleting it, then you can avoid downtime. I use the following script (shamelessly taken from the lazyweb elsewhere and adapted to my needs):

oldpool=pool-1
oldnodes=$(k get no --selector='cloud.google.com/gke-nodepool='$oldpool -o json | jq .items[].metadata.name -r | xargs)

kubectl cordon --selector='cloud.google.com/gke-nodepool='$oldpool

for n in $oldnodes; do
  echo "draining $n"
  kubectl drain --delete-local-data --ignore-daemonsets $n
  echo "----------------------------"
done;