I have Avast 2014.9.0.2021 and don't want to change the program version at this time. My registration expired, so I tried to use offline registration. I filled out the web form and waited for the email.
When it arrived, I followed the instructions and pasted the key into the form but received an error "<license key> is not a valid license key". I double checked that I only copied the 1 line in the middle. I reattempted to register 3 times. Same error message each time.
Looks like offline registration is broken. If I had to guess, I'd say you probably are assuming everyone is using the current version of Avast - but that's not a valid assumption.