Max Tech has already posted a tear-down video. Which yielded the first two surprises we didn't expect : not only is the SSD socketed and therefore potentially replaceable (sadly it looks to be a bespoke socket), there is another socket inside for a second drive. Currently I'm on a works' PC which blocks YouTube so i can't pop a link, but feel free to search his recent videos (depending when you look it's currently the most recent).
As for replacing the standard drive with a larger aftermarket version, maybe the 'bios' on on the Mac will prevent you from booting from a third party drive, but if so, I wouldn't expect it will take people like OWC and Crucial long to crack this, along with the bespoke socket. The second socket is easily accessible after just removing the base, but the built-in SSD takes a little more disassembly to access, but nothing like stripping-down an iMac.
As for replacing the standard drive with a larger aftermarket version, maybe the 'bios' on on the Mac will prevent you from booting from a third party drive, but if so, I wouldn't expect it will take people like OWC and Crucial long to crack this, along with the bespoke socket. The second socket is easily accessible after just removing the base, but the built-in SSD takes a little more disassembly to access, but nothing like stripping-down an iMac.
