QUESTION: Can I upgrade YDL from one version to the next?

Generally speaking, you may use yum to upgrade from Yellow Dog Linux X.y to X.0.z (ie: 4.0 to 4.0.1) and usually from X.y to X.z (ie: 4.0 to 4.1). However, we do not support nor advise the upgrade between major distribution revisions such as X.y to Y.z (ie: 3.0 to 4.0). As it is far simpler to backup your /home/ dir (which you should do on a routine basis anyway), reinstall Yellow Dog Linux, and then migrate your /home/ dir back into its proper directory (making certain to use the same username on the reinstall).