Snort is at version 2.9.3.1.1 and is constantly being developed to integrate new and more powerful features and detection. The newer features that have been added, as well as most of the rules developed to combat present day threats, will not work with older versions of Snort. The VRT writes detection content as the Snort team writes code features, both of which evolve Snort to help you defeat these threats. This always leads us to recommend that you should perform a simple upgrade.”]
Source: https://blog.talosintelligence.com/2012/09/the-best-defense-is-good-defense.html