Digium's ISO that uses the builtin Asterisk GUI interface, very different from FreePBX and you can not go back and forth, but may be a good option for low powered embedded devices that are inadequate for a FreePBX environment.
http://www.asterisknow.org/index
Note: AsteriskNOW 1.7.1 is based on CentOS 5.5 and now uses FreePBX 2.7 by default.
If you want to upgrade to a later FreePBX version you need to do the following change:
Edit your /etc/yum.conf file and add the following in the [main] section:
exclude=freepbx*
This change prevents yum from downgrading FreePBX and destroying your install when you run yum update
