This was not my understanding of how a Creative Commons license works.
The licenses say specifically “The licensor cannot revoke these freedoms as long as you follow the license terms.”
Please correct me if I am wrong, but my understanding is that, once you license and publish (i.e. upload to iNat) content licensed as Creative Commons, that version of the content is forever licensed thus. Nothing stops you from re-licensing a work and making any subsequent changes to that work be unlicensed for public distribution (i.e. all-rights-reserved), but the version that you originally published as CC will continue to be available for use under that license, and it’s only subsequent modifications that will remain yours only. You can make the latter explicit by using the CC-BY-ND (no derivatives) license.
With this in mind, all downstream data aggregators such as GBIF can use those original observations that one licensed as CC however they see fit (as long as they respect the terms of the license).