Add a new bcrypted password column. Alter authentication so that it uses bcrypt column if populated. Alter registration/password changes so that it populates

Add a new bcrypted password column. Alter authentication so that it uses bcrypt column if populated. Alter registration/password changes so that it populates bcrypt by default. As old users log in (and you therefore have access to their plain text password and know that it's valid) populate the bcrypt with the correct hash. After a period of N weeks when you've basically migrated all of your active users, do a forced password reset on the rest. Kill the MD5 column. Done :)