It may be hard to give you a concrete yes/no answer, because things always can go wrong, and even if you get the battery replaced, you may break something in the process of getting it put back together. (The replacement procedure is quite an involved ordeal, as you are likely aware).
That said, I found this forum which caused me to dig into it a little deeper. There is software that allows you to pull the music off of you iPod - such as CopyTrans, that may allow you to back up your iPod, if you so desire to do so. In fact, here is a whole list of software that should provide that functionality for you. Scroll up on that link, and you may find more helpful information.
Back to the point: replacing the main battery shouldn't have any negative effect on your iPod. (One interesting side note: notice Apple never warns about leaving a fully discharged device too long with the risk of losing data - there are other problems that may occur if that was to happen, of course). iPods use Flash storage which is not dependent on constant power - like some types of RAM are.
So the short of this: you shouldn't lose data, but back it up anyway.