You can do this manually, but you're probably better slapping it in a plugin in the long run ( just hook into function AfterUserDelete() ).

Basically, when you delete a user, replace the author_ID in comments with null and populate comment_author_name/email/url

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thank you for that, but another question comes to mind after i hear your solution. How can i then find out if any comments left in my system are from users that no longer exist and if they don't exist in the system anymore can i replace those users with null and everything will be ok? Or will i break the system?

There's probably some fancy sql you can run, but basically you need to compare your current user ID's with comment user id's and update any records that no longer have a current user.

If it was me I'd NULL the user ID, and change the details to "anonymous", "dummy@email.com", "" ( no need for url ) for all the non-existent users

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Last month I deleted 1 user, and after deleting all posts of that user, at the end the deletingprocess told me 'I converted all comments to visitor comments'...
so you don't have to worry, it's build in