Once there are a critical number of refueling stations along the major highways, then the number of NG vehicles will increase dramatically.
Also, existing LNG trucks are produced by taking a diesel truck and applying an expensive conversion kit. Once there's enough demand, the truck manufacturers will produce trucks that start as LNG trucks, greatly reducing the price.
There will be a basic LNG supply chain on the Interstate system by the end of this year. It should be more than 200 stations by two different companies at strategic locations. Just a start, but enough to start
I would say most of the initial are not change outs but rather new trucks in fleets like UPS and others.