avast doesn't apply a version limit on its licenses, if there is a program update during the license period it is valid for that version also.
avast is unlike many AVs and doesn't have an annual version number change forcing users to upgrade/purchase again.
avast is unlike any program I have ever used where there is an incessant march to higher version numbers and when might rate a 0.1 increment some others would call a major update at either 0.5 or even a whole ordinal 1.0.
I have had avast for almost 6 years and have only travelled from version 4 to version 5, yet there have been massive changes in functionality and performance.
The license is good for one system or however many you bought for, if you have a redundant system and get a new system install on the new system and use the license and get rid of the old system installation of avast.