Changing slide masters in Keynote
I've been asked this a couple of times, and it's not terribly hard - so let's pretend that you have a slide deck from 'Conf 12' that you want to re-use for 'Conf 13'...
First off, you'll need a Keynote template for 'Conf 13'. Lots of events won't supply that (no idea why not... I do...) which you'll probably get as a 'pot' or 'potx' powerpoint templates. To open that in Keynote, simply rename to ppt or pptx and it'll open straightaway. Now, in Keynote, check the fonts sizes etc look correct, then choose File, Export, and save it as a Keynote template. It will by default suggest the Keynote template directory:
Next up, reopen your 'Conf 12' slide deck. Select all the slides, click on 'Themes' and then select your 'Conf 13' theme - if the original slide was written correctly (ie, you used the placeholders) then it should all change over as expected. Probably bar the title slide. Select the title slide, then go to 'Masters'. Drop that down and you'll see that BOTH the Conf 12 and Conf 13 masters are active. Choose the Conf 13 title master and that should change too.
Voila! Now all you need to do is update the content ;-)