They have, a few times.
When Colorado originally launched, it had only one exit (the basement) but it was later patched so the basement was only mandatory on your first playthrough.
Hitman 2 also made a variety of changes to older maps such as implementing the foliage to hide in, as part of that new system.
There’s also a LOT of far smaller incremental changes that have been made over time between entries.
I do wish we had the option to turn that on and off, because I would leave it on. I love playing the games like a story and it feels incomplete when you can’t finish it like how it was the first time. Colorado, for example, originally got rid of Diana’s dialogue when you enter the basement room, then put it back but the original music stayed gone. You also don’t get her comments on the face scanner anymore. In Berlin, the Agents’ info doesn’t populate the way it did the first time, like you’ve never encounters these guys before.
Every time I play, I want it to be like it was the first time again, and while I understand that a lot of people don’t want to have that each time, giving us the option to turn it on and off would be welcome. Like Diana’s introducing the target; enough with the 2 day limit, I want to hear her say it again if I have to reload from just before she said it, and those who don’t can turn the switch off.