You can add or remove resources, and toggle between revisions of those resources as long as you are working with a library in Development. If your library has been moved out of Dev, you'd have to first reject it from its current environment and move it back into Dev which will enable you to make changes again. This is meant as a way to help streamline the publish workflow, but it has some major inconveniences. However, it sounds like Adobe is making improvements to this and they're working on a rapid rollback functionality. I think this thread may be relevant to your question: https://forums.adobe.com/ideas/10273
