as! vs as operator in Xcode 6.3 in Swift

Question

Swift changed a lot with Xcode 6.3. I had to replace dozens of places in each of my app as -> as!. Why, what are the rules now?

Answer 1

According to the release notes:

The notions of guaranteed conversion and “forced failable” conversion are now separated into two operators. Forced failable conversion now uses the as! operator. The ! makes it clear to readers of code that the cast may fail and produce a runtime error. The “as” operator remains for upcasts (e.g. “someDerivedValue as Base”) and type annotations (“0 as Int8”) which are guaranteed to never fail.