You can only Opt-Out of the inclusive access program through the deadline listed on the Course Materials page in your course. After this deadline, you can no longer Opt-Out.
You can Opt-In again up until the census date, which could be later than the deadline to Opt-Out. As long as you see Opt-In as an option on the Course Materials page, you can Opt-in again. Once the census is complete, this option will be removed and you will no longer be able to Opt-In or Opt-Out.