Alexis80,
You can use either an SATA HDD or an SATA SSD. Both types are compatible. It has to be a 2.5" drive (laptop size), of course. The space for and the SATA adapter for adding an SATA HDD/SSD is under the lid.
1. The transfer is not automatic. You can clone the onboard storage to the new HDD/SSD, then change the first boot item in the BIOS to the new drive. You can see the official method for cloning the OS to a new drive on
this forum post.
2. Though I can't give an official answer on this, as I'm not an ACEPC employee, I've had no problems upgrading to the new version from Microsoft itself. I always perform a clean install from USB installation media that I create using Microsoft's Media Creation Tool rather than upgrading in place, though I'm sure upgrading in place would be fine too.
HOWEVER, two things I recommend, and which I ALWAYS do before performing a clean install of the new version (or upgrading in place, for that matter), is first,
saving a system image of the existing install in case anything goes wrong, so I can go back to that if I need to, and second,
saving the Driver File Repository of the existing installation so that I have all of the drivers I need for the new install, just in case any ACEPC specific drivers are needed. This is, of course, AFTER setup of the initial system. IOW, I didn't perform the upgrade on my AK3 until I had performed the setup of the initial OS version it came with, both so I could first make sure that the system activated and in order to save the two things previously mentioned. Mine came with version 1909, and after the initial setup I performed a clean install of version 2004. I have yet to upgrade to 20H2, but I will do that soon, and again with a clean install.
ACEPC does have Windows 10 Pro 64 bit version 2004 available for the AK3. You can get it
here. The installation tutorial is
here.
3. The admin or someone with experience with a Linux distro on an AK3 will have to answer this.