Ask HN: Have the HN downvoting rules changed recently?

I just noticed that I can't downvote any more. I've been able to downvote ever since I hit 200+ karma points, so am slightly confused why I can't downvote when I am at 400+.

I also noticed that the interface also recently started showing my avg next to my karma. Have there been recent changes to HN that I've missed? A massive influx of new users leading to the downvoting threshhold being inflated? Something else?

Whilst I don't care about karma levels, I believe downvoting has been a necessary tool to maintaining the quality of the HN community. Any insights would be very helpful. Thanks!