I Think that you have to. you may be able just to pull the head off and work on it while its in the bike. when I pulled both of my quad motors they didn't have too much room to work with though so Idk if you can do it while its in the bike.
....All in all if you want it done right.... take it to RSR mechanical shop they will cut it open and replace everything with the proper duct tape that is required.
you "CAN" do it by tilting the engine, but that comes from very few people that know the bike VERY WELL. If this is your first piston change on the DS, I'd suggest just pulling the engine. That is how I did mine (in Jan) and it wasn't too bad. Timing kicked my ass (please search threads that I started for tips/tricks), but the engine removal was easy. Just take your time and follow the manual.
Yeah, there's no magic to it, but the DS is one of the most unconventional quads I've ever come across. There are many parts and pieces that are interdependent. One part has to be removed to get to something else, so on and so forth. I'm sure if you have mechanical aptitude and a fair amount of patience, you can save yourself the expense of having a shop complete the task...and, of course all of us on the forum are here to lend a digital hand. However, if you want peace of mind, have a pro do it.