You can also use a standard ATX PSU that you can purchase online from any store.
As far as warranty is concerned, its as the HP agent mentioned in the other thread. HP configured the Obelisk to fit standard (non proprietary) parts in the case. The big deal is that HP usually uses some parts that are not replaceable with standard stuff, but not with the Obelisk. That's all it means.
But if you do replace anything yourself, HP will not cover the PC at that point. So yes you can upgrade with standard parts now, but know that if you do, its at your own risk, not HP's.
Im just a volunteer and here to answer the questions about the PC itself. I'm not going to argue over HP Policy because I have no authority to change that.