diff --git a/deployment/nova/nova-compute-container-puppet.yaml b/deployment/nova/nova-compute-container-puppet.yaml
index 902fd838c6..482b28b900 100644
--- a/deployment/nova/nova-compute-container-puppet.yaml
+++ b/deployment/nova/nova-compute-container-puppet.yaml
@@ -432,7 +432,7 @@ parameters:
   NovaHWMachineType:
     description: >
       To specify a default machine type per host architecture.
-    default: ''
+    default: 'x86_64=q35'
     type: string
     tags:
       - role_specific
diff --git a/environments/nova-hw-machine-type-upgrade.yaml b/environments/nova-hw-machine-type-upgrade.yaml
new file mode 100644
index 0000000000..cfd5f6cbd3
--- /dev/null
+++ b/environments/nova-hw-machine-type-upgrade.yaml
@@ -0,0 +1,10 @@
+# A Heat environment file to pin NovaHWMachineType and in turn
+# [libvirt]hw_machine_type to the legacy default machine types during
+# an upgrade. This allows operators to then ensure a machine type is
+# recorded for existing instances *before* changing to the new
+# default. See the following Nova documentation for more details:
+#
+# https://docs.openstack.org/nova/latest/admin/hw-machine-type.html
+#
+parameter_defaults:
+  NovaHWMachineType: 'x86_64=pc-i440fx-rhel7.6.0,aarch64=virt-rhel7.6.0,ppc64=pseries-rhel7.6.0,ppc64le=pseries-rhel7.6.0'
diff --git a/releasenotes/notes/nova-machine-type-default-changed-27244a925f6d6200.yaml b/releasenotes/notes/nova-machine-type-default-changed-27244a925f6d6200.yaml
index ed237f81e1..56ffaf630a 100644
--- a/releasenotes/notes/nova-machine-type-default-changed-27244a925f6d6200.yaml
+++ b/releasenotes/notes/nova-machine-type-default-changed-27244a925f6d6200.yaml
@@ -1,6 +1,17 @@
 ---
 upgrade:
   - |
-    The ``NovaHWMachineType`` parameter now defaults to an empty string with
-    per architecture machine type defaults instead being provided directly by
-    OpenStack Nova.
+    The ``NovaHWMachineType`` parameter now defaults ``x86_64`` based instances
+    to the unversioned ``q35`` machine type. The remaining architecture machine
+    type defaults being provided directly by OpenStack Nova.
+
+    A ``environments/nova-hw-machine-type-upgrade.yaml`` environment file has
+    been provided to pin ``NovaHWMachineType`` to the previous versioned
+    machine type defaults during an upgrade.
+
+    When the upgrade of the overcloud is complete the following OpenStack Nova
+    documentation should then be used to ensure a machine type is recorded for
+    all existing instances before the new ``NovaHWMachineType`` default can be
+    used in the environment.
+
+    https://docs.openstack.org/nova/latest/admin/hw-machine-type.html#update