Please see the version 1.7.2 post for safely updating from 1.7.1: https://www.spigotmc.org/resources/multichat.26204/update?update=271095
The 1.7.3 update is just a small extension to 1.7.2 to fix a previous issue with Sponge formatting: https://www.spigotmc.org/resources/multichat.26204/update?update=271204