Question: I recently installed Ubuntu 12.10 and it requires a passphrase to boot up (I installed it with an encrypted file system).

Do I have to reinstall to change to a standard unencrypted file system?

Answer: 
In case it is OK to keep the encryption, but to switch off the passphrase prompt, a much simpler approach is to just set a trivial password like "password" and then save that trivial password in the initramfs in cleartext. [Disable the LUKS encryption password](http://atterer.org/linux-remove-disable-luks-encryption-password-on-disk-partition-crypttab-initrd).

Essentially, add a hook script which in turn adds a "keyscript" to the initramfs. Usually these scripts are used to get the password via Bletooth, from a USB stick etc., but in this case, just make it print the trivial password.
It is actually possible to decrypt the partition in place without much effort. For example, see the instructions [here](https://wiki.archlinux.org/index.php/Removing_System_Encryption), which are generally just as valid for Ubuntu as for Arch. In my case, I had a LUKS1 device, which apparently makes things easier. All I had to do was this:

1. Boot into a live environment using a USB stick. I used Ubuntu 18.04.
2. Run `sudo cryptsetup-reencrypt --decrypt <device_path>`.

That was it. For a 250 GB SSD, it took 20 minutes. I didn't have to do anything special to `/etc/fstab`, grub, or initramfs. I commented out the relevant (only) line in `/etc/crypttab`, but I don't even think that was necessary.

That said, I had a second machine with a 500 GB SSD, and after about 3 hours, it still claimed it was going to be another 90 minutes and the rate was only getting slower, so I gave up and just reflashed the drive.

Before doing anything, though, I recommend backing up the (decrypted) partition. Using dd [as described in another answer](https://askubuntu.com/a/412737/173694) is great. I did it while the partition was mounted before I rebooted into the live USB environment. This came in handy for the partition I ended up reflashing, as I was able to remount the backup and copy over anything I cared strongly about.