I worked at Quora, but left before this change was made, but I believe it was totally retroactive, mainly because I got emails with information about my previous anonymous answers and a deadline to get the one-time link.

Now... if the emails were logged and in the exploited database, then all bets are off, but there's no indication that happened at all.

There are about a hundred other things about this that give me anxiety, but Quora is run by extremely competent people (engineering and otherwise), so I am pretty confident about their ability to be transparent and to know the extent of any issue.

This entire thing is really shitty for everyone involved, but given Quora's tenure (almost nine years!) that this is the first breach is pretty amazing, and that they've done so much work to make it less of a problem is great.

None of the above is meant to diminish the general dissatisfaction others are expressing here.