Your iPhone 7/Plus case will fit the new iPhone 8/Plus
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Apple lists all of its new iPhone 8 and 8 Plus cases as compatible with the iPhone 7 and 7 Plus
Not only Apple's, but also all third-party wrappers from Otterbox and others are also listed as compatible with both phone generations, so if you have been thinking of keeping your favorite case you now have on your iPhone 7, and upgrading to the iPhone 8, for example, you will probably be able to fit the new phone in the old case without the help of a heat gun.
Things that are NOT allowed: