If you're an top-level account admin, and there are enough courses that you need to do it in a bulk, you can also do this through a SIS Import. Create a csv file with and include the account_id column in addition to the required fields, or use the output of a courses provisioning report. You can find more info on the required fields/format in the courses.csv section of the SIS Import Format Documentation - Canvas LMS REST API Documentation
If I recall, you'll put the SIS_ID of the destination sub-account into the account_id column. If you manually created the sub-accounts then they won't have a SIS_ID by default, and you'll need to add one in the sub-account settings in the interface before proceeding.
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