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Verified with versions:
nmstate-1.0.2-11.el8_4.noarch
nispor-1.0.1-4.el8.x86_64
NetworkManager-1.30.0-7.el8.x86_64
openvswitch2.13-2.13.0-105.el8fdp.x86_64

OVS vlan tag can be changed:

---
dns-resolver:
  config: {}
  running:
    search:
    - rhts.eng.pek2.redhat.com
    server:
    - 10.73.2.107
    - 10.73.2.108
    - 10.66.127.10
route-rules:
  config: []
routes:
  config: []
  running: []
interfaces:
- name: ovs_br0
  type: ovs-bridge
  state: up
  bridge:
    options:
      fail-mode: ''
      mcast-snooping-enable: false
      rstp: false
      stp: false
    port:
    - name: dummy0
    - name: net
      vlan:
        mode: access
        tag: 102
  lldp:
    enabled: false
  ovs-db:
    external_ids:
      NM.connection.uuid: 7952ed45-538b-43ce-8c0c-1751833fe4ac

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (nmstate bug fix update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2021:2589