Saw a Reddit post talking about this scene from 1937's Sh! The Octopus:
It is 82 years old and a great practical effect. How was it shot, though?
I thought this was going to be a long and difficult investigation...
However, the answer was right there on IMDB, in the Trivia section
The transformation scene was done in much the same way the beginning part of the transformation was done on Dr. Jekyll and Mr. Hyde (1931). The actress was made up in exaggerated highlights and shadows (including her teeth) using a single color shade of makeup. When filmed through a filter the same shade as the makeup, it's invisible to the camera. Take the filter away and the makeup suddenly becomes visible. Besides a filter, the makeup can also be hidden by similar-colored light. This type of trick was only possible on black-and-white film.
Very clever indeed.
The seemingly 'slow' transformation is then probably achieved using a graduated filter [filter at one end, clear at the other], larger than the lens, which can be pulled relatively slowly in front, to achieve the apparent change.