It should not be possible to repaginate a quiz with attempts, but you can through the settings form

Steps to reproduce

1. Create a quiz and add 3 questions, all on one page.
2. Attempt the quiz as a student.
3. Go to the Questions (Edit quiz) page. Note that you cannot change the pagination, because the quiz has been attempted.
4. Go to the quiz settings, and expand the Layout section.
5. Change 'New page' to 'Every question'. Notice that the 'Repaginate now' checkbox becomes ticked.
6. Click Save changes, the go to the Questions (Edit quiz) page.

Actual result: The quiz has been repaginated.

Expected result: the quiz should not have been repaginated, and therefore, the checkbox on the form should not be present, or should be disabled.

Originally reported in this forum thread: https://moodle.org/mod/forum/discuss.php?d=432684.