Ok. I've just found out how to replace 2 scripts in a lib file. I used a hex editor to catch alterations PXLEditor do when you save your lib with scripts.
Results:

1. The 6th byte in the lib represents a number of total files;
   
2. The byte right after script's name represents the size of the script.

So if you want to replace 2 scripts not copy-paste-renaming, you should use an hex editor replacing the bytes wich represents script's names and the bytes of actual text. But that's actually much harder to do than copy-pasting!

So, if coming autumn term in the university is not hard for me, I'll write a prograame for Windows which allows easy and fast replacing of files in libs.