Upgrading a major C++ compiler version was never free in my experience, but even when working in a codebase with ~2M LOC the upgrade (e.g. 14 -> 17) was something that could be prepared in a set of feature branches by one person over the span of one, maybe two weeks.

That greatly depends on your project, what dependencies it has, and what's involved in the migration. For example, I recall a previous project I worked on that experienced a considerable amount of non-trivial issues when upgrading to C++14 due to unforeseeable curve balls. One of them was caused by a third-party dependency toggling constexpr versions of its member functions only on C++14, which caused a bunch of obscure linker errors as old symbols were no longer available.