If you decide after you create a slide that you want it to have a different layout, you can easily change the layout by displaying the slide and then, on the Slide Layouts tab of the Elements Gallery, with Apply To Slide selected, clicking the layout you want.

You can make space on a slide for additional content by changing the slide layout to one that supports more content, or by adding a text box or graphic object. For example, if you want to add another layout placeholder to a Title And Content slide, you can do so by applying the Two Content slide layout, from the Slide Layouts tab of the Elements Gallery. You can add any graphic element to a slide, independent of a layout placeholder, by inserting it from the Insert menu. PowerPoint rearranges the existing content to fit the new layout.

PowerPoint rearranges the existing content for you when you apply a different slide layout.

You can move a layout placeholder on a slide by pointing to it and then, when the pointer changes to a four-headed arrow, dragging the placeholder to its new location. You can change the size of a layout placeholder by clicking its border to activate the resize and rotate handles, and then dragging the resize handles. If you don’t like the change you made, you can reset the layout placeholders (while still preserving any content you’ve entered in them) by reapplying the original slide layout.

You can see both the original placeholder content and the changed content while dragging the resize handles.