I tried the following:
1. Disable "auto update" for Maps in Google Play
2. Uninstall Maps 7.4 with Titanium Backup
3. Install Maps 6.14.4 from the APK file
This works... until I restart the phone. Then 7.4 is immediately back again. Is this to do with the NAND protection on the HTC One? How can I make the change permanent?