First time I've heard of someone wanting to underclock a LiteOn and not overclock it!
Theoretically, you should be able to do it since it's a Mediatek series 5 chip that the 48x and 40x share. 40x LiteOn's will flash up to the 48x firmware if done properly.
But a BIG word of warning...this is a risky proposition, that could lead to a bad flash and a fragged drive. It has to be done in DOS with specialized software as well. You will assume all risks involved with this!!!
If you are still interested after all this ya, contact me privately for a link on how to do this.