A company that I worked at had a few very large C++ related migrations, and they were all very very expensive.

The first was removing `long` from the code, since a lot of code assumed its size (is it like `int` or like `long long int`?) and as machines were upgraded it caused problems.

The second was moving to C++11/14/17. Most of the difficulty was toolchains on unixen that did not support the new versions of the language, or for which support was incomplete, or for which upgrading to a version with support broke existing builds.

The third was moving to Linux from big iron unixen. As far as I understand, this initiative is still underway. It was already underway in 2011 when I joined the company.

This is a rich company with a large, healthy engineering department. I imagine that most other companies would not or could not bother.