@Mike,
You are right. Try this query (please try it on a test site first OR save your database first).
UPDATE mdl_lesson_pages INNER JOIN
(SELECT mdl_lesson_answers.*
FROM mdl_lesson_pages INNER JOIN mdl_lesson_answers ON mdl_lesson_pages.id = mdl_lesson_answers.pageid)
AS ExistingPage
ON mdl_lesson_pages.qtype = "3"
AND mdl_lesson_pages.id = ExistingPage.pageid
SET mdl_lesson_pages.qtype = "20";