The full backup is not "overwritten" technically but it is injected with the oldest incremental to maintain your retention policy. What this means is if you set your retention to 14, then once the 15th incremental runs, the oldest incremental will be injected into the full backup to bring your restore points up to 14 to comply with your retention policy setting. There is some more information here on Backup Retention
Also, the backup retention policy counts backups not days. So if you set your retention to 14 take 3 backups, then turn your computer off for 15 days, when you run your next backup it would be restore point #4 and not inject the full or remove any incrementals until you created your 15th restore point, which then would inject the oldest and move the full forward. This is called the Forever Forward Incremental method.
If you have any other questions please ask!