How can one reverse this expantion?
Read this thread for more information:How to expand the storage memory on the Yún
From the second post in that thread: [quote author=Federico Fissore date=1398843712 link=msg=1702476] If you encounter any error and would like to restore your yun to its previous state, all you need to do is to reboot it with the SD card UNplugged, then edit file /etc/config/fstab removing the following section
config mount option target '/overlay' option device '/dev/sda2' option fstype 'ext4' option enabled '1' option enabled_fsck '0' option options 'rw,sync,noatime,nodiratime'
and reboot once more [/quote]