Modifying Custom Post Type after registration (will it affect content?)

No, it will not. It changes just the interface in the post editor: you get the page attributes box to select a parent.

All existing post have already a value for the parent: 0. You can find it in the post_parent property of each post. The only thing that changes now is the ability to change that value.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)