Typically, you can't reuse many parts when you switch processor technologies; if you change chips, you change the entire motherboard at the same time. That won't be true for AMD in the future, though. It's working on a common chip framework, Project Skybridge, that will let 2015-era ARM and x86 system-on-chip processors share the same pin layout. In other words, a basic motherboard design could handle both CPU types.
Via: AnandTech (1), (2)
Source: AMD