Honestly, I find this concern more compelling than concern over what Francis has actually said about the death penalty.
I mean, ultimately, he’s the pope. So I’d be hard pressed to say that he can’t do this. But it is sort of uncharted territory. A 2nd edition of the Catechism came out a few years after the 1st and there were several changes, but that was mainly because there was now finally an authoritative Latin edition. We haven’t had any pope make any changes to the Catechism of his own initiative. This is a first.
Generally, when a pope makes changes to canon law (which does happen often enough), he will release some sort of official motu proprio . But Francis did not do that here. He just sort of said “Here’s the new paragraph” and the CDF wrote a letter of explanation to go along with it. It seems like a pretty informal way to go about it. But the Catechism is a different sort of thing than canon law.