In JIRA 6.x, 7.x, or 8.x, it is possible to rename a project key. However, it is not possible to reuse a project key that was previously used in Jira, even if the original key has been changed.

Even archiving a Project, Jira will still keep the history of already used keys.

Current Behavior:

• Let's say the user has renamed a project key from "ABC" to "KLM"
• Although there is no project that uses the project key ABC, the user is not allowed to re-use "ABC" for the new project key.

Suggested behavior:

• Ideally, the user should be able to re-use the project key "ABC" for other/new project, once the project key has been altered to "KLM".

Workaround

• Currently, in order to re-use a project key you need to perform one of the options listed in Editing a Project key: Either delete the project KLM to free up the ABC key, or Export the project and then re-Import it. 

DO NOT use SQL queries to change DB tables to try to re-use keys.

This will lead to database inconsistencies as projects are connected to issues, attachments, permissions, workflows, plugins, custom scripts, and etc..