Help with MP3/WMA file conversion for my new iPod?

Okay, so I have ALOT of music on my laptop in MP3 or WMA formats. Got a new iPod (80GB clic). If I start to load iTunes and try to load all of this music (from laptop) to my iPod, does it automatically replace the old MP3/WMA files with converted AAC files? Or does it just make a new copy? I really don't want to lose the MP3 or WMA formats on my laptop just to use the new iPod. Any suggestions? Thanks & Happy Holidays.