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