Can I go back to a non-backports version of my machine?

I still want ultimately to have the latest version of Digikam but revert everthing else, including Amarok to version 1.4 again.

Is it a matter of disabling the backport repo and doing 'sudo apt-get upgrade' ?

It's not that simple, because you'll have installed newer versions of the packages than are available in the standard repo. apt-get upgrade (or any variant on that) will not upgrade packages to "older" packages just because you don't have a valid repo for them The most straightforward way of fixing it is to reinstall. You could use apt-cache policy and iterate over all your packages, working out which came from backports and which from the normal repos, then manually downgrade all those packages, but it'll take a fair while to do if you've pulled in all of KDE. In general, you should use pinning when using a backport, to make sure that you don't inadvertently pull in lots of unwanted packages. https://help.ubuntu.com/community/UbuntuBackports has notes on how to do this.