It is more common for the "D" light to burn out first, but in your case the "L" light went before the "D".
There are detailed procedures and discussions both on Edmunds Town Hall RX300 forum (there is a "search" you can use to find it) and also on Club Lexus RX300 first gen.
I've been meaning to get into mine to replace the "D" indicator, but the display is redundant anyway, since cars didn't used to have a display on the dash anyway, so it hasn't bubbled up to the top of any of my to do lists. My 1999 RX also needs the "ECT Power" and "ECT Normal" bulbs, but I don't think you have those on the 2003 model.
Other owners have reported that it wasn't too difficult, but it's a time-consuming job to pull the instrument panel. I've done this on other cars, and I'd do it again myself if necessary.
Join the forum to share your experience with other owners, and if you do go ahead and replace it.