demo.yaml

---
kind: Service
apiVersion: v1
metadata:
  generateName: echo-service-
  annotations:
    annotation-key: annotation-value
  labels:
    label-key: label-value

overlay.yaml

#@ load("@ytt:overlay", "overlay")

#@ def has_generate_name(index, left, right):
#@   return not("name" in left["metadata"]) and "generateName" in left["metadata"]
#@ end

#@ def set_name(new_name):
#@overlay/match missing_ok=True
name: #@ new_name
#@overlay/remove
generateName: ~
#@ end

#@ def generate_name(left, right):
#@   return overlay.apply(left, set_name(left["generateName"]))
#@ end

#@overlay/match by=has_generate_name, expects="1+"
---
#@overlay/replace via=generate_name
metadata:
  #@overlay/match missing_ok=True
  annotations:
    #@overlay/match missing_ok=True
    kapp.k14s.io/update-strategy: "always-replace"