After sleeping on this, I've realized a critical flaw in my thinking - who says Amazon will keep paying for the current SIM after 3G is shutdown? So I'll probably need to do a SIM swap as well.
Also, I've come up with an alternative scheme - replace the 3G modem with a USB flash drive for extra storage (4GB is
tiny nowadays) using a mini-PCIe to USB 2.0 adapter/riser like in
this picture. Though how exactly would it be mounted in the filesystem is unknown to me currently.