This is, quite literally, a perennial problem with Avast. I have just had my own reminder that I have to re-register next month and I'm dreading the possible hassle as by then I'm likely to have two new PCs to register as well.
The last time I did a manual re-registration but the time before I had to do a full re-install of Avast because the flakey re-registration process mysteriously installed the trial version of the full package they were pushing at the time. I couldn't get rid of it (the forum tools that are available now weren't then) and I ended up installing a very old version I had on a magazine disc over it. Registered that without any trouble and then updated it to the then current version.
I really don't want that sort of trouble again and this post does not encourage me that what should be a simple process is going to be smooth and trouble free.
One thing I would say is that I seem to remember in the past the re-registration process apparently wouldn't work before a certain date (not the expiry date). Of course sending e-mail reminders about re-registration a month in advance with no warnings about that meant people reported similar failed re-registration problems when they tried to do it immediately on receipt of the reminder.