Free job boards tend to allow for jobs to be present for approximately one month. Even before that, your job may begin to slowly lose traffic. In that case, you may wish to repost your job onto the job boards in order to ensure visibility and gain more candidates.
The best option is:
1. Close each Job.
2. Duplicate each Closed Job.
3. Bulk move candidates from the Closed Jobs to the new Jobs (if applicable).
This will make sure that your Jobs stay on top of the job boards' feeds again. Reposting the same Jobs will gain you less traffic, as the job boards have already registered them as old Jobs.