I hit up this question in the review queue and reopened it.
Even though there may be mostly an answer in the "duplicate"the "duplicate", it's hard to find and doesn't focus on the particular problem at hand. Reading through the "duplicate", it talks about having a style guide with templates and conventions and guidelines for using the infrastructure and tools (which would include compilers and languages).
However, this "duplicate" doesn't address updating these conventions as the environment changes or how you manage software that was written as these conventions apply. I don't think the current answer does that well, either. I'd like to see an answer to the new question about good practices for maintaining and revising the things mentioned in the "duplicate" and how to have legacy code under a previous revision of the conventions live with new code written under a current revision of the conventions.
These questions are definitely related. I think the top answer in the "duplicate" is worth reading. But once you have those things which help manage large and complex source code, there's no guidance for revising as time goes on. I think that's what is being looked for in the new question.