Everything including the venting was replaced. The vent was upsized from 3" to 4", the vent material was originally 30ga metal and was replaced with 26ga metal. A rise off the top of the tank before the first vent elbow was also added.
The entire water heater, gas line, and water lines were moved back about 18" to reduce the horizontal vent span. Moving the tank also made more space in the mechanical room.
A thermal expansion tank was installed and set to 73 psi. The expansion tank was strapped to the wall. A new PRV was also installed in the entire house. I'll go ahead and say it, this is a rockstar installation by KC Water Heaters.