How to change master of an existing slide in Keynote 6.0?
Ah! A few more minutes of "seeking" - and I found out where Apple moved the functionality. 

For all desperate - here the solution:


Make sure you clicked on the slide-overview (on the left), so that the slide gets a yellow border

Then far-away on the right select the "Format" button. Then below it you will see the current master slide for the selected slide. With a small grey triangle on the right. Click this triangle and a pop over will allow you to change the master of the current slide.



Hope this helps someone in the future...