I'm a bit more cynical about the name changes. Once the public figured out what "Progressive" meant (in the mid-20th Century), the term became something of a pejorative. So the leftists switched to "Liberal." Then the public figured out what this new misuse of "Liberal" meant (in the 1970s and 80s), and "Liberal" became a pejorative, too.
Since much of the public has a short memory, the leftists are now switching back to "Progressive."