Your slide show will not necessarily be improved by picking one slide and updating it. Nor will updating two or ten slides. A slide show will be stunning when the level of the entire deck has been raised. A slide show will be stunning when it represents your message elegantly.
The dangers of changing one slide are when the designer of that slide does not take into account the other slides in the deck. It is not unusual to see one beautifully sparkly slide that seems to be out of place with its peers. It is normal for several updated slides to be inconsistent with the style and theme of your slide show.