I agree with @eskimo: whatever you're asking for seems like it's not something you really want.
unsafeBitCast and friends rely deeply on compiler behaviour, and in general you don't want to get too far into making assumptions about the structure layout algorithm.
In this case you doubly don't want to do this because
Any? may hold a reference type, which means it is itself a potentially reference counted object. That means that running a bit cast or similar approach is likely to go badly. In principle it's ok to do if you never use the A any longer, but it'd be better to just ask for it to be turned into a B, which you can do by writing the appropriate initializer.
The better thing to do is to write an initializer: