Except it’s not really free. Or you could say it’s not really an upgrade. It’s actually an “opportunity” offered to current users to replace the (genuinely) free version of AVG with the trial version, which eventually you’d have to pay for if you wanted to keep it.
I’ve used AVG Free myself and recommended it to all of my clients for many years now but they do have a nasty habit of trying to maneuver you into getting the paid-for version. I’ve absolutely no objection to them offering it from time to time. If they couldn’t do that there would be no free version, which is just a “loss leader”, to pull you in, as far as they’re concerned. It’s the fact that you have to be on the ball to avoid being suckered into it that I object to.
Anyway, as long as you are on the ball, or at least remember to up your game and get on it whenever you’re being offered something by a corporation, it’s well worth having, so here’s a link. But at the time of writing the prominent download button on that page is for the trial version, which is the one you don’t want! (Unless of course you do.)