An LFS replay is not just some recorded "film" of what occured, everything is re-created every time you choose to play a replay. Whenever LFS undergoes updates or improvements to the physics the older replays become obsolete. It has always been so.
You can copy your entire LFS to new folder (on the same hard drive as your current installation) and install an older patch (whichever one dates back to your replays), and it should work AFAIK, though it's been a while since I had the need to do this.