Xcode 9 & Xcode 8.3.# both versions supports Swift 3.2. If your existing project has default language Swift 3.2, then it will work in Xcode 9, with minor changes. But if your project has support of Swift version 3.1 or lower then you must migrate it into 3.2 or 4.0, to work with Xcode 9

Project ► (Select Your Project Target) ► Build Settings ► (Type 'swift' in Searchbar) Swift Compiler Language ► Swift Language Version ► Click on Language list to open it.

Convert your source code from Swift 3.2 to 4.0. For easier migration of your code, follow these steps: (it will help you to covert into latest version of swift supported by your Xcode Tool)

(Xcode) Menus: Edit ► Covert ► To Current Swift Syntax

I hope this may help you