With Swift 2.x only being available in Xcode for a short while longer, how about revisiting the conversion topic again? 😀 This article by Chengyin Liu details the work to get AirBnB converted from Swift 2.x to 3, and also covers some of the loose ends that they haven't quite wrapped up yet. The article mentions that they had a 5½ week schedule for the work but don't mention how long it actually took. I'd be interested to know.
Update: I had a message from Paul @ AirBnB who clarified and said that the work completed on schedule in 5½ weeks. Thanks for the update!
Then there's the case of Artsy who adopted Swift super early and then decided it wasn't right for their app. They started conversion from Swift to React a year ago and this article gives a great summary of the process and tradeoffs.
One thing strikes me about both these stories... We knew it would happen but the introduction of Swift back in 2014 has put a huge burden on those people/companies who adopted it early. So much lost time!