Switching disks is actually done the same way as inserting a disk the first time, except, instead of pressing enter then selecting a file to run from the disk, you press the function key to select the highlighted disk image. The screen will indicate, at the bottom, which function key that is. Just watch for "Select Disk" at the bottom.
I think you can also select the disk, then, when the directory is visible, just press Escape a few times to return to the emulation with the new disk inserted. That should work, too, but I usually do it the other way because, although it requires a little more reading, it's still a little quicker.
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