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: 
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.
@pattivacek 's short answer worked fine for me.
To get rid of the start screen "Enter your password" part - i just added

```
apt purge cryptsetup

```

after that.