I would not work from the ISO image. This will require you to install it as a
separate distro instead of as a desktop. All you need to do is install it from
Synaptic and it will appear in the sessions menu on the login screen. This way
it will use the same root partition and home folder.

Load Synaptic and go to Meta Packages (multiverse) and install
mythbuntu-desktop. Same goes for Ubuntu Studio, except it is in Meta Packages
(universe) and install ubuntustudio-desktop. Doing it this way you will avoid
all sorts of potential problems.