Taken from /r/MoneroSupport:
How can I rename my Monero wallet in the GUI or CLI?
Can I just rename the wallet file, the *.address.txt file, and *.keys file?
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Renaming the files is just fine. The wallet files have no idea what they're called, so no conflicts will happen. The FOO.address.txt file can be deleted, it's just information for the user, and is not used after creation. Just make sure the files aren't being used (ie a wallet program running) while you rename or the files will be saved again with the old name when the wallet program exits.
In the CLI, I've done this, and you can just rename those three files to a new name. Not sure about the GUI.