The Change-Master-Slide dialog defaults to a selection of the currently-used master. But once one clicks any other master - it is no longer possible to tell which master is the one being used right now (except by looking outside the dialog; but the dialog may hide a lot of the slide area). I suggest that some visual indication of the currently-used master be used, beyond the default choice of selected item (and distinguishable from it). The absolutely simplest option is to append something like " (current)" to the master slide name at the bottom of the tile view. I would like something more noticeable though. This would be particularly important if bug 164015 were to be addressed, and the currently-used master would not always be the default.
We should highlight the selected item (usually a bluish background) and use the current frame indicator for the active master.