Migrate iPhone 3.1.3 app to iOS 4 compatibility (screen size and resolution)

I've developed an iPhone 3.1.3 app and I want to make it compatible with iOS4.

Do I have to do something special?

iPhone 3GS screen is 480x320 px and 163 dpi. iPh开发者_如何学编程one 4 screen is 960x640 px and 326 dpi.

Do I have to change my images resolutions and views?

By the way, I've used iPhone 3 SDK. Now I'm going to install iPhone 4 SDK.

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You don't have to do anything. iOS will scale the images for you.