Keep in mind, just because they are based off the prusa i3, an open source printer, doesn't mean they are open source. I started out with an i3 kit that turned out to be closed source. And some of them use custom control boards. A good chunk of the big 3D printing companies are open source, prusa, aleph objects (lulzbot), bcn and to some degree ultimaker. On the other hand, most chinese manufacturers don't want to or don't bother publishing source files.

To upgrade its print area, you'd have to replace just about the whole thing. To upgrade all axis (and I don't recommend this much, mendel style printers perform worse and worse as they get scaled up). To just upgrade the X axis, you'd need a new extrusion for the top and bottom of the frame, new extrusion for the x axis, new print bed, new y carriage plate (and with a bed that wide, i'd recommend two y rails to make it more stable, new belts, wires and bowden tube. I'd also recommend a dual sided z axis drive for an x axis that wide.