how do I change an assignment page to a content page?
Sorry there's no easier way, but they are two completely different beasts within Canvas. There is no way, short of making API calls, to do it automatically.
P.S. When I say API calls, I mean an API call to fetch the assignment text, an API call to create the content page, and possibly an API call to delete the assignment. All those need done within a script that can carry the information from one API call to the next, not something that can be done with a single API call.
Retrieving data ...