I looked at the
iFixit site for that and yes, its a lot easier than on the iMac (the only glue involved is tearing off the rubber feet to get at the screws) but obviously not a "user serviceable part" so bye-bye warranty. You'd be talking about a SATA-based M.2 drive rather than Apple's super-fast 4xPCIe drives (and it sounds like the HD slot could be SATA 2 only). Not ideal on a machine that only really competes with the iMac on price if you cost in a nice Wacom tablet for the latter.
As for the GPU, well, I'm in the UK and the surface studio isn't available here yet, so by the time it materialises we could have an iMac with a bumped GPU.
The MS surface range does have me curious (they look more
fun than the current Macs, and I'm not scared of Windows), but I haven't liked what I've heard when looking deeper.