One thing I just saw looking up more information is this line:

"For [travel outside the US], your ticket is valid for one year. If travel was started, your ticket must be reissued and all travel must be completed within one year of the start date. If travel was not started, your ticket must be reissued and travel must begin within one year of the original date of purchase."

So I guess this means I would have until the end of Sept 2024 to have my return flight from Europe completed, subject to change fees, if I wait until after my flight begins, but if I cancel beforehand, I would have to at the very least have a new ticket by mid-January, potentially with travel completed by then (depending on how it actually seems to work)?