I won't re-write what Google said but Roger has done another excellent job catching and writing about a question all webmasters have, every time they lose traffic in an update. Worth reading - [searchenginejournal.com...]

My opinion - I don't believe there are algorithmic penalties at all. Just thresholds, collected signals, and time. Go ahead and change the URL of all your categories and you won't get penalized, but, since they were the strongest pages linking to your content and they are now reset, your traffic will be reset too for 5-6 months(repeatable). The data gathered on the old category URLs won't benefit the articles anymore.

Thresholds aren't only in search, they are on Youtube too. Create an AWESOME video and post it to a new youtube account and you'll cap out around 350 views. Give it to a more trusted account and it may get millions of views. The difference is not an "algorithmic penalty". It's just thresholds, collected signals, and time.

If you get hit by a core update, did you really get hit or just end up on the other side of a threshold? Again, not a penalty. The data collected about your site that allowed you into higher traffic levels didn't change but the threshold did.

What you want to be careful of is a page or site reset, then you're waiting 5-6 months to get traffic again no matter what you do. Too much spam, naughty backlinks, but also things like changing category URLs have the same effect.

Once you wrap your head around the fact you're trying to send the right signals, and are waiting a set amount of time, to get to a higher level of traffic per article you won't sweat the bumps as much, or risk taking shortcuts that may reset.