I use copy and paste alot when editing the files. I also put a comment next to what I changed and why, so if I did something wrong I can at least find the lines I edited. When commenting make sure you use // before the comment starts, and put the comment at the very end of the line. This helps out when its a large edit.

I am not sure if you wanted to, but Ludens said you wanted late era units available from Early. If that's the case I think you are going to have to tweak the Units file as well to make the late era units available in early. Even though you have a Master Bowyer doesn't mean Pavise Arbalesters are going to be available in Early. The unit has a time restriction placed on it as well.

I am not much of a modder so I hope I've helped. Make sure you make copies of the original unmodified files. If you did really wreck the file, I know I have, you can at least back up to the original file and start from scratch.