A transformation was a change from one form to another.

In 2269, James T. Kirk noted in his captain's log that "Argo was once a land planet but its surface is now almost completely covered by water. The change was caused by violent seismic disturbances. Our mission is to study the effects which the quakes and other phenomena had on its surface. This knowledge may save millions of lives on a Federation planet identical to Argo which will soon be undergoing similar transformation." (TAS: "The Ambergris Element")