Attach new page to existing navigation menu item and inherit existing slug

You can edit the slug of the original and the new /about pages in the WP Editor.

But how about the first part of the question

How can I attach a new page to existing menu without having to create a new menu item?

Hi Alice,

edit the slug on both the pages - so make the original About something else. Then change the new about slug to About.

If the menu breaks then go to the Appearance > Menus and add in your new About link to the menu.

Alice if your question is because you are concerned about SEO or Google you can redirect the old page to the new one using a plugin like: Redirection

Thx!

You're welcome.