The so-called “Codex Chimalpahin” contains the following passage (vol. 3, f. 29v, line 15 onwards):

auh ca nimã ye yc huitz. omocencauh omochichiuh yehuatl ynitoca copil. ca cenca huey tlahueliloc. Auh ca cenca huey nahualle amo mach iuhqui yn inan yn itoca Mallinalxoch. ca cenca huey tlahueliloc. yn copil. niman ye huitz. ypan ce calli xihuitl 1285. años. oncan mocuepaco. yn itocayocan Çoquitzinco. ye no ceppa huitz. oncan mocuepaco yn itocayocan atlapalco. ye no ceppa huitz. oncan mocuepaco yn itocayocan ytztapaltemoc. Auh ca yehuatl yn copil. yc mocuep ypan moquixti ytztapaltetl. yc motocayotia. yn axcan ca tiquitohua yn mochi tlacatl. ytztapaltetitla Auh ca in yehuatl yn copil. ca ynecuepca mochiuh yn itztapaltetl. in yehuatl yn copil.

Here’s how Anderson & Schroeder translate it:

And thereupon the one named Copil came; he prepared himself; he adorned himself. He was exceedingly wicked and a very great nahualli. Copil was not the equal of his mother, Malinalxoch by name, but [nonetheless] was exceedingly wicked. Then he came in the year One House, 1285. He turned back to the place called Çoquitzinco. Once again he advanced, turned back to the place named Atlapalco, once again advanced, turned back to the place named Itztapaltemoc. And when Copil returned he appeared as a paving stone (itztapaltetl). Hence the place is named and now every one of us says Itztapaltetitla, and Copil’s return was as a paving stone.

They apparently decided that nahualle was an error for nahualli, even though those kinds of mistakes aren’t typically found in Chimalpahin.

However, the word nahualli has at least two meanings. The most common is “sorcerer” or “wizard”, i.e. a wise person with various magic powers. That’s presumably what Anderson & Schroeder intended. But a less common meaning is “a form that someone can magically transform themselves into”. For example, the Florentine Codex refers to inaoal, inecuepaliz, inenextiliz in tezcatlipuca “the nahualli, the transformation, the manifestation of Tezcatlipoca”. Inecuepaliz “his transformation” is from mocuepa “to turn oneself [into]”, a word with similar polysemy to English turn.

From that sense you get nahuale “one that has a magical form”. For example, Chimalpahin mentions in his Sixth Relation tequannahualeque “those with dangerous animal nahuallis“. But shortly after they’re introduced, hualquiz atl yc aocmo huel mocuepque, yn izqui tlamantli yn innahual “the water came out so they could no longer turn into any of their nahuallis” (whatever that means).

Chimalpahin very frequently writes a double -ll- where a single -l- would be expected, and sometimes vice versa. So nahualle in the passage quoted above is more likely to represent the word nahuale “one that has a magical form”, rather than the word nahualli “sorcerer”.

Once you accept that Copil is being described not as a guy with generic magic powers, but specifically as a shapeshifter, that helps put the rest of the passage into context. Although the word mocuepa (and its various forms) can mean “turn back, return”, here it probably means “transform”. Instead of:

And when Copil returned he appeared as a paving stone (itztapaltetl). Hence the place is named and now every one of us says Itztapaltetitla, and Copil’s return was as a paving stone.

I would translate it something like: