Looking at the last example in section 3.2 here:
https://access.redhat.com/documentation/en/red-hat-openstack-platform/9/paged/upgrading-red-hat-openstack-platform/chapter-3-director-based-environments-performing-upgrades-to-major-versions

I see: "diff -Nary /usr/share/openstack-tripleo-heat-templates/ ~/templates/my-overcloud/"

Trying that (limiting to 10 lines output) - it produces too much output.

[stack@undercloud ~]$ diff -Nary /usr/share/openstack-tripleo-heat-templates/ ~/pilot/templates/overcloud |head
diff -Nary /usr/share/openstack-tripleo-heat-templates/all-nodes-validation.yaml /home/stack/pilot/templates/overcloud/all-nodes-validation.yaml
heat_template_version: 2015-04-30                               heat_template_version: 2015-04-30

description: >                                                  description: >
  Software Config to drive validations that occur on all node     Software Config to drive validations that occur on all node
  Note, you need the heat-config-script element built into yo     Note, you need the heat-config-script element built into yo
  images, due to the script group below.                          images, due to the script group below.

parameters:                                                     parameters:
  PingTestIps:                                                    PingTestIps:



I'd suggest to use diff -Nar /usr/share/openstack-tripleo-heat-templates/ ~/templates/my-overcloud/

Here's an example:
[stack@undercloud ~]$ diff -Nar /usr/share/openstack-tripleo-heat-templates/ ~/pilot/templates/overcloud |head
diff -Nar /usr/share/openstack-tripleo-heat-templates/environments/storage-environment.yaml /home/stack/pilot/templates/overcloud/environments/storage-environment.yaml
5d4
<   OS::TripleO::NodeUserData: /home/stack/wipe-disk.yaml



Just the relevant output.