You can change a slide’s master at any time. For example, you might want to change a slide featuring a photo from a horizontal to a vertical photo master.
Any style changes you’ve made to the slide (for example changing the font), are retained when you change its master, but you can remove the style changes by reapplying the slide master.
Select the slide whose master you want to change, click Masters in the toolbar, and then choose a different master slide.
Select the slide and choose Format > “Reapply Master to Slide.”
Select the slide whose layout you want to change.
Choose View > Show Inspector, and then click the Slide inspector button.
Click Appearance.
To apply a different slide master, click the slide thumbnail image and choose a master from the pop-up list.