In order to delete your Game Sync ID, we would need to confirm your registration. Please provide the following information by submitting a question at support.pokemon.com:
- Trainer Club account username
- PGL nickname
- Registered DOB, starting with the year
- The country you live in
- Current Game Card in use
- Currently registered Game Sync ID
When deletion of the registered Game Sync ID is completed, we will reply to your question. Then please register your new Game Sync ID.
Source Good luck getting it deleted!