So in your case, you could add a prefix 'txt$' in front of plain text passwords, and when the user successfully logs in, have it hash the password and save it in the database.
The advantage of doing it the way Django does is that the "upgrade on successful login" lets you change your hash without causing problems with the users.
Give the users a bonus or some reason for logging into their account and most of your passwords will get fixed automatically.
It has a prefix for the hashing algorithm at the beginning, so it can check which algorithm is being used for each password:
sha1$a1976$a36cc8cbf81742a8fb52e221aaeab48ed7f58ab4
So in your case, you could add a prefix 'txt$' in front of plain text passwords, and when the user successfully logs in, have it hash the password and save it in the database.
The advantage of doing it the way Django does is that the "upgrade on successful login" lets you change your hash without causing problems with the users.
Give the users a bonus or some reason for logging into their account and most of your passwords will get fixed automatically.