Retool

A retool is a newer version of an existing toy which has had parts either added, removed, or resculpted. This can be as simple as changing a single part to provide stability or ease of transformation, to altering a large percentage of the toy's appearance or gimmickry.... In rare cases, extensive remolding even changes some aspects of a toy's transformation.

Some refer to retooling as remolding, though this is not necessarily accurate. This implies an entirely new mold be made for the toy, while sometimes the parts for both figures are produced on the same sprues. In the case of Universe Rumble and Inferno, the only difference was the head, and both were on the original mold sprues for Scrapmetal. What they did with all the Cobybot heads is anyone's guess; they might have a warehouse full of 'em somewhere.

From Teletraan I: The Transformers Wiki, a Wikia wiki.