There's an update for Xcode 6.3 and it includes a new version of Swift (1.2). If I update, do I have to migrate all my Swift code to 1.2 or can I select an older version of Swift?
You have to migrate all your code to Swift 1.2 if you're using Xcode 6.3. If you go to Edit > Convert > To Latest Swift Syntax, it will help find many of the changes. However, there's still a lot of manual editing to do.
In the long run, it will serve you better to keep your Swift code at the latest version so that future migrations are easier and so that you can take advantage of future enhancements to the language.
You can install multiple toolchains for Xcode to use - wrote a small how-to here: https://medium.com/@pardel/using-old-versions-of-swift-in-xcode-4dd46644a257
TL;DR - Download & install old Swift toolchains from: https://swift.org/download/#releases