The problem is the ‘incremental’ virus definitions update can’t handle the difference between what is in the database in the old installation file (be that avast5 or avast6) and what the current virus definitions database contains now.
There aren’t separate databases for avast 5 or 6 or 7, it is just the content of the database at that point in time.
So when there is a massive gap in what was in the database at that time and now, it simply recommends that you download the ‘full’ database at this point in time rather than attempt an incremental update.
The only suggestion is to do a manual download of the full database (a friend/library, etc. saved to USB) but that is quite large. But at least it will bring it up to date, after that a periodic incremental update should be possible.