You're best off leaving it as an original LC...not only because it was your first computer, and you probably have a lot of great memories of the machine...thats one of the reasons as to why my LCIII (which I've had since I was 8) still runs its original logic board, despite me having a spare LC475 board in an ESD safe bag. Not to mention that yes, you will have to use a new case bottom if you put any other model logic board in there. The original LC used an integrated fan/speaker unit that mounted to the case in a different way to the seperate fan and speaker the later models had, and you must swap in a seperate fan and speaker from a later model when doing the upgrade.