You will not loose your library, Simply install pro over your basic and it will behave just as before only better, and will take over all the previous preferences. Your files are not stored in Limewire anyway. If you check in the library by highlighting a file and clicking explore you will see where the file is.
A word of warning. Never use the delete function on a file in the library unless you want to delete it instantly, as that is what will happen
Be sure to get Limewire pro from
http://www.limewire.com/english/content/home.shtml
I hope this helps