The fda parameter can be used to define a disk when starting QEMU.

$ qemu-system-x86_64 -fda floppy1.img [..]

If you get to the point where you need to swap disks, enter CTRL+ALT+2 and update the fda disk. Or so the online guides say.

(qemu) change fda "floppy1.img"
==> Device 'fda' not found

Recent versions of QEMU seem to have changed this. Run info block showed a different identifier for the floppy drive:

floppy0: ./floppy1.img (raw)
    Removable device: not locked, tray closed 
    Cache mode:       writeback

Using this identifier works.

(qemu) change floppy0 "floppy2.img"
==> [silently accepted]

Now you can go back with CTRL+ALT+1.