OP gave off a strong impression of really not being sure about how to open them and fix them, therefore me linking to the guide that shows everything about how relatively easy of a fix it is to make.
You don't risk losing any parts unless you really don't bother taking the minimal precautions required to just, you know, not put the small parts in risky places. Put them in a bowl or something if you're afraid of knocking the pieces off your desk, and I guess you can glue down the bowl if you're afraid of accidentally knocking that off the table too.
On a more serious note though, I guess the main risk here in potentially losing those parts is if you're doing the repair on your computer desk near your keyboard. Just gotta keep it in mind so the disk and rubber button don't fall off into the keyboard ;p
KipMudz's guide doesn't really show this off but gaining access to that part of the board is really easy and you don't really risk breaking any components off the board (no ribbon cables or anything like that attached to bottom shell, etc.)
https://www.ifixit.com/Guide/Nintendo+DS+Lite+Trigger+Buttons+Replacement/4757
Also if you -did- lose those pieces for whatever reason,
DealExtreme has replacements for like $2 at best. No soldering required, you can just open the replacement switches the same way you would the ones on the board and take their metal disks/rubber switches haha; unless you legitimately broke the switches off your board, in which case you'd need to get new ones soldered on (to be fair though, you wouldn't lose much if that happened anyway, since your shoulder buttons already don't work, so it's worth trying.)
