Because this wasn't a surprise Apple just dumped on people, this was a well established by the open Swift community.
The core language itself has only really had minor tweaks, how the standard Apple libraries are referenced is what's caused the major change (e.g. not so much the language was horrible in v2 but awkward naming conventions from bridging the Objective-C way of doing things) as well as some naming changes for Swift standard libraries.
Presumably if they went with Swift it was for an iOS/OSX project where Objective-C was the alternative. I find I'm much more productive in Swift than Obj-C, and time spent migrating Swift 2 to 3 for me is dwarfed by the time I saved using Swift.
So to answer your question: If the upgrade doesn't take long he should take a pay cut because: this wasn't a surprise that jumped on them, this was a well known and documented set of upcoming changes that shouldn't take much time to implement - especially given the (not always perfect) migration tools provided.
If the OP wants to say that everyone else is wrong all the time and get a pay raise when he's proven right then he should get a pay cut if he's proven wrong.