serenum.sys is the serial port enumerator, part of the system for communicating over serial ports.
I'd have to windiff it, and learn a bit about the file format to let you know what the differences are, but I bet there are some minor differences in there preventing the old code from working (probably either security enhancements, bug fixes, or changes in how things work to prevent some sort of memory issue or buffer issue). I bet the reason the new version isnt supported is that whoever is responsible for dealing with the code for that isn't trying to keep up.
I would also say that unless your using serial communications in other programs, that it would probably be safe to replace the file. However, I wouldn't suggest you do that unless the computer is expendable, or at the very least you can reformat and reinstall windows on it without a problem. If if the system is strictly for programming PLCs and such, and you've got all the licensing backed up and al the rest of that stuff, it should be fine.
