I'm looking to test out a whole bunch of distros and desktop environments.

I've got about 20+ dvd's with different distros and desktop environments and I'm trying to find the best combination of distro and desktop environment.

Is it possible to use a live distro and change the desktop environment, or am I stuck with whatever the default desktop environment is until after a full installation?

I've seen distros that have multiple iso's with different desktop environments. Is it possible to change the desktop environment of a distro iso before burning the disc, so I could burn live multiple distro dvds with all the supported desktop environments to test out?

Multiple distros on a live iso requires rebuilding the live iso to include all of the DEs you want to check out. It's also possible to boot a live distro with one DE (for example, XFCE) and then download and install a second DE (say, LXDE). The second DE obviously disappears when you shut down, but it's worked for me (with the above example).

http://distrowatch.com/table.php?distribution=hybryde

Is multi live already.

There was another with 48 or so different Window managers and maybe Desktop Enviorments but I don't remember the name at the moment.

Edit: Found it. http://distrowatch.com/table.php?distribution=linuxbbq

You could save yourself a lot of DVDs if you installed VirtualBox and booted each iso in VBox. You can make changes to the Desktop by downloading and installing but with the same caveat as above, gone on reboot.