It is not currently possible to change the project key through JIRA's interface. The best way to do this is to:
- Export your JIRA data to XML.
- Go through the file and replace the instances of the project's key:
- in the 'key' attribute of the Project entity
- in the 'key' attribute of all Issue entities.
- Search for the project key in the whole XML file. You should not find too many references. Change any that you find.
- Rename all attachment folders, as the folder name depends on the project key. This includes one folder for each issue with an attachment, plus one top-level folder for the project.
- Reimport your data.
- Reindex the data by navigating to Administration -> System -> Indexing and selecting 'Re-Index'.