The specific details will depend on the app. Some apps can use relative layout controls to stretch or contract to handle the space well. Others will need to change the layout by showing or hiding different control sets, moving controls around, etc.
As an example, the wizard based grid apps swap their GridView for a ListView when in Snapped mode. Portrait view is also a likely candidate to require a different layout.
See
Guidelines for scaling to screens for more information.
--Rob
