Hi Howard!
My apologies for the late reply!
Related to your question, once somebody has created a wiki page with a format, it's not possible to change it However, this student can create a new page with the correct format, and link it from the main page.
If all the students have to use the same format for their pages, I would recommend you to use the "Force" param: https://docs.moodle.org/en/Wiki_settings#Wiki_formats
Cheers!
Sara