Both ways are done. However, if you want your floor to be WARRANTIED by the MFGR., I'd be SURE I follow their instructions. I'm going to bet that the MFGR. wants the 1st idea used. IMO, that's best. HOWEVER, you could rip up the 'old' stuff, and depending what's down there, that might satisfy the MFGR's Warrenty requirement. If you want to rip up what's there, first I'd find out what you have for a subfloor and then check w/the MFGR to see if it's warrantable. Remember, the MORE layers you add to what you have MAY cause 'transition problems' to other rooms in the bedroom, or to the hallway, or to the like. That's why it's better, sometimes, to go to the subfloor and build up the new floor to the CORRECT level.
As an aside, I want to kinda warn you that floating floors are 'noisy'. Since they float, there's a hollow sound when you walk on it. Worst, the rooms below can hear EVERYTHING! Personally, if you're planning on laying area rugs down, I'd just carpet the room. 1) You don't spend THAT much time walking on the bedroom floor. (Think about it.) 2) For the $$$, you have to weigh the expense. I like hardwood but NOT on a second floor, or above, or in the kitchen. I'm a hardwood floor guy that likes it in the foyer, living room, family room, and dining room. That's pretty much it.