As others noted, it’s darn near impossible to get a pick into the seam to get started opening the case. I finally got mine by pressing up on the back of the case, which causes the side of the back piece to angle out a bit and allowed me to get the pick that came with the replacement battery in. Once I finally got that far, it was fairly easy to slide the pick up and down the side of the case to pop free each of the clips. I scubbed up the edge of mine a bit before I figured this out, and those efforts didn’t help, so maybe avoid that. (I didn’t have an Exacto like others have suggested, but having done this, I can see how that might work well also, if you are careful about it)
Note that the lock will move out of position again easily once you have the case open (when disengaged from the internal switch). That being said, you will want to keep this little switch in mind even beyond making sure the lock is in the right place, as the little plastic tab that is the switch is extremely small and fragile. I ended up breaking mine off somehow while trying to get the case closed. It is small enough that I never felt it break; I finally got the iPod back together and found that the lock switch didn’t work. I was able to re-open it and use a pick tool to unlock what was left of the switch, so my iPod works, but no longer locks. I’m happy to have a working battery anyhow, but I’d have been happier yet to have completed the repair without breaking anything - be careful with this little switch.