Can I make my apple store app free with ads and give previous buyers ad free add-on?
Yes and there's a few different ways to do it.  The path I followed was to release a small update that use NSUserDefaults to write a bool property when the app was installed.  Then, when I distributed the app update with the ads, I'd read if the user had that bool value.  If they did, they were a previous owner and I wouldn't show the ads.  If not, they were a new owner and got the ads.

You can do the same thing by having the app set a value on your own server (the app would call your server upon installation with a user-specific string).  This would do the same as using NSUserDefaults except the value is stored on your server and not the user's device.

There's a few examples online including how to store the bool value that will stay with the user even if they delete and reinstall the app.