Depends on the approach. There's an odd benefit to C++ being the starting language; C++ is ancient and because of that you can make it interop with most other languages; in this case, the switch to Swift will probably simply mean replacing C++ modules (likely starting with the ones that cause the most memory bugs) with Swift modules and gradually switching things out since Swift has C++ interop[0].
It's less a "rewrite from the ground up" and more a "this is the main language we'll be using from now on".
As for the reason to stop using C++; honestly what reason isn't there by now. The only reason to still use C++ in this day and age is if you're targeting very non-standard hardware or are developing videogames. The language is memory unsafe in a way that almost every other language isn't (like, even if you don't pick a language with an overly worrywart borrow checker, C++ is remarkably worse at memory management), the standard library is awful and it's stuck with the chain and ball of being a superset of C on it's leg, meaning that every reason why you wouldn't use C also applies to C++ (minus the "C isn't OOP" one).
It's less a "rewrite from the ground up" and more a "this is the main language we'll be using from now on".
As for the reason to stop using C++; honestly what reason isn't there by now. The only reason to still use C++ in this day and age is if you're targeting very non-standard hardware or are developing videogames. The language is memory unsafe in a way that almost every other language isn't (like, even if you don't pick a language with an overly worrywart borrow checker, C++ is remarkably worse at memory management), the standard library is awful and it's stuck with the chain and ball of being a superset of C on it's leg, meaning that every reason why you wouldn't use C also applies to C++ (minus the "C isn't OOP" one).
[0]: https://www.swift.org/documentation/cxx-interop/