If your plan expires, your learners' progress will be saved but their access will be downgraded to audit mode. This means that learners will no longer have access to locked assessments or be able to obtain the verified certificate. To complete the course and obtain the certificate, they would have to pay a fee for that individual course.
However, if you purchase a new plan and invite the same learners to the program, they will be able to continue where they left off after they re-enroll.