It's not possible for users to delete a site and then for the URL to be freed up. When a user deletes a site, the site is flagged as "deleted" in the database, but not totally removed, so the URL is still reserved.
It's possible for me to delete a site altogether, making the URL available. In this case, I would recommend instead changing the URL of the old site, to ensure the migration. So:
1. I modify https://jjcmpa.commons.gc.cuny.edu/ so that the URL is https://jjcmpa-legacy.commons.gc.cuny.edu/ (or something like that)
2. The JJMPA team creates a new site at https://jjcmpa.commons.gc.cuny.edu/ and migrates content over
3. The JJMPA team marks https://jjcmpa-legacy.commons.gc.cuny.edu/ as private, since it's only for historical purposes
If it would be helpful for the team, I can help to streamline this a bit:
1. I modify https://jjcmpa.commons.gc.cuny.edu/ so that the URL is https://jjcmpa-legacy.commons.gc.cuny.edu/ (or something like that)
2. I modify https://mpaprogram.commons.gc.cuny.edu/ so that the URL is https://jjcmpa.commons.gc.cuny.edu/ (in other words, URL musical chairs)
3. The JJMPA team marks https://jjcmpa-legacy.commons.gc.cuny.edu/ as private, since it's only for historical purposes
Step 2 is quick for me, and would save the team some trouble. But it means that the site at mpaprogram.commons will be gone (because it will have become jjcmpa)
Let me know how you want to proceed.