When deleting an issue type, you have to keep in mind that the issue type might already be in use, meaning that issues have already been created with that issue type. So, when you delete an issue type, you will need to select a new one for those issues. The good news is that Jira takes care of this for you. As we can see in the following screenshot, when we delete the Bug issue type, Jira informs us of the already existing 13 issues that are of the Bug type. You will need to assign them to a new issue type, such as Improvement: