Universal or iPhone Question

Things will be different if you switch to Universal from iPhone (displayed on an iPad). It depends on how you laid out various things. And the iPad uses pop-overs rather than full screen modal displays so things change (e.g. you don't call viewWillAppear after you dismiss a modal).

But try it in Xcode and see what happens. It's a test of your programming skills how many changes you will need to make and whether they are straightforward. It makes you appreciate "MVC" programming.

>Is it exceptable to just click the arrow on the iPad to make it fit the screen ?

If you are talking about 1x/2x mode, often, yes, but not always. Be sure to test, at least on the simulators (speak up if you need that process) so you can see what App Review sees. Or did App Review already issue a rejection on that issue?

If you want to properly support iPad, yes, you should convert to a Universal app. But doing that will not automatically convert it's layouts either...you'll need to test your app for yourself to see where the problems are, problems with various solutions depending on your exact layouts.

See the docs to begin that process: https://developer.apple.com/design/human-interface-guidelines/ios/visual-design/adaptivity-and-layout/