These standards support a gradual migration strategy. Agencies can continue to deploy their current technologies until a new standard is in place. When the new standard is approved, they'll replace their existing readers and tags in the normal course of events when these devices wear out. As soon as the standard is complete, I envision they'll stop buying their old tags and start buying new ones. On average, tags need to be replaced every 5 to 7 years.
Readers have a longer lifetime.
Interoperability requires much more than just a DSRC standard. One example of interoperability would be that you could drive your car on different toll roads in different states and they would all bill your VISA card. To you, the driver, the overall toll-road system would be interoperable. However, the toll roads would need to make sure that their financial systems all work in a collaborative fashion so that your charges are all billed in the same way. Our goal is to allow manufacturers to design in backward compatibility so that they can support existing DSRC applications, but not to write backward compatibility into the standard.