https://github.com/kwsch/PKHeX/blob/5708310f6f075dd29b9b28e5131c2b9a676cb881/PKHeX.Core/Saves/SAV3.cs#L51-L54
https://github.com/kwsch/PKHeX/blob/5708310f6f075dd29b9b28e5131c2b9a676cb881/PKHeX.Core/Saves/SAV3.cs#L304-L354
1. Download a hex editor (ex. HxD)
2. Based on the structure of Gen3 save files, you need to locate where the Trainer Name is stored. Referring to PKHeX's code, the OT Name is stored at the start of the Small structure block, occupies 8 bytes, and is immediately followed by [byte Gender, byte Padding, uint TIDSID]. If you can find your Trainer ID at any offset (0x*00A), with data roughly matching the playtime, try changing the first 6 bytes (0x*000 - 0x*006) to be 0xFF. Do this for the backup save as well, so it'll need to be done at another offset with the same style.
3. Load in pkhex, should load as SAV3EU, export. Should be fixed.