dban is irrelevant for this purpose and just time consuming. The disk started out encrypted, so the data is secure enough that there is no reason to use a secure erase, particularly since the same data will be placed back on the disk without encryption.
If you want to save the data on the disk, you must create a BACKUP before proceeding. If not, then the easiest approach is to shove in the install DVD, boot on it, and follow the prompts to reinstall withOUT encryption.
If you ARE planning to save the data, then you'll probably want to do something like this:
boot on a livecd and run cryptsetup to map the encrypted partition to an decrypted device file, then backup the decrypted device file to a separate location using something along the lines of "dd if=/dev/mapper/whatever of=/mnt/whatever/backup" or "dd if=/dev/mapper/whatever | gzip > /mnt/whatever/backup.gz", then UNMAP the encrypted partition and run "fdisk /dev/sda" and change the partition type to NON-encrypted linux. Then restore the backup directly to the partition "dd if=/mnt/whatever/backup of=/dev/sda2" (presumably its the second partition on the first disk??) or "zcat /mnt/whatever/backup.gz | dd of=/dev/sda2".
After this is done, there should also be an additional step needed in order to specify that it should use the partition directly rather than mapping it, but I'm fuzzy on the details of this part.