SE editor in chief here. What you describe is incorrect. The only thing we do is very light sound-alike spelling modernization, like "to-night" -> "tonight". We do not do things like change from en-GB to en-US, replace old words with different modern words, or change text for "American readers", whatever that means. I have no idea where you got that impression.
I personally worked on the Forsyte saga. If you think something was done in error, please let us know and we'll be happy to fix it.