There is some old version support - just virus definitions updates, not entirely sure if program updates would be done on historic versions.
The last released version (build number) of the old major versions, 6, 7, 8, 9, etc. would have limited support - however, your 7.0.1426 isn't the last avast 7 build, that was 7.0.1474. That may have been the trigger for the update to the next avast version 8.0. 8.0.1506
Interestingly it has switched you to:
8.0.1506
2014-01-27
Support version for migration to Avast 2014
This as it mentions is a build that essentially checks for the ability to support migration to avast 2014. So I don't know if sometime in the future you might end up on avast 2014.