So will you be able to easily change the SSD?
It seems unlikely. This appears to be the only site currently offering a (partial) view of the inside of the Pixel. The instructions on disassembly have been taken down "temporarily", but until an enthusiast dismantles one this the best chance to find out if the SSD is integrated to the mainboard or is a removable module. There are chips visible in the photograph, which seem likely to be RAM (though I can't find the exact number anywhere online).
EDIT: It appears that there is a single flash memory chip just to the left of the USB ports, with a circular white sticker on it. If the SSD is indeed on the motherboard, you would only be able to upgrade with external storage. An MSATA card cannot be used to upgrade because the mini-pcie port that holds the WLAN card is not wired for this. Some discussion, here.