⟩ Will the new design mean I have to completely relearn how to use my phone?
Not really. Launching and using apps is largely the same. But there will be some new things to learn. For example, you can now swipe up from the bottom of the screen to summon Control Center (a handy panel that gives you access to many of your most commonly-used system features). There’s another new swipe-from-the-left-edge gesture that means “go up a level” when you’re in Mail, for example.