The OP can't downgrade, but he/she can uninstall Avast and install v.6 and it will work. They will get the same virus definition updates. Just set your GUI Settings > Updates > Ask.
Yes,
Actually you can go back to version 6 at any time, if you have it on your computer in your Downloads Folder, or wherever you put it.
You can get the Avast Uninstall Utility, boot into Safe Mode and uninstall ALL Avast versions.
Than after you get Avast version 6 on your computer, Register for your license. Than simply go under update settings, and have the Engine and Virus Definitions set to Auto, so you will get daily definition updates.
BUT
Set Update Program to Manual, and you will not get any notice about updates to any new versions. However, you can still open up the GUI, and update the program to later versions when you feel confident that they will be OK.
Jack
PS. There are people still on version 5 and I think version 4 is still floating around out there. Not sure if version 4 is still being updated with new definitions, but version 5 is.