When you install Ubuntu along side Windows, Ubuntu will be added to Windows boot manager. Futhermore, when you boot up your computer, windows boot manager will show up since you have two operating systems to select and the DEFAULT OS is disabled.
When you uninstall Ubuntu (research that), it should remove itself from Windows Boot Manager and windows will automatically select Windows 8 as the default OS.
Ubuntu will not replace bootmanager, since Windows is the main partion, ubuntu is similar to a seperate hard drive which can be easily plugged in and out.
So to your answer, you will not need to reinstall Windows if you want to restore the default boot manager.
WINDOWS BOOT MANAGER WITH UBUNTU: http://i.stack.imgur.com/XapQj.jpg