AFAIK it is not possible to refresh the JWT without disconnecting and reconnecting. In fact, the examples in the link seem to actually disconnect MQTT and reconnect.
When I tested (mOS 2.17 during 2020), I didn’t see errors but timely disconnect/reconnects every hour (IIRC).
Please check if 2.17 behavior is different for you.
Some developers have been working on some cloud stuff and that includes GCP (IIRC); but I don’t know whether that includes JWT refresh stuff nor if nor when it will be released.