Hello! If I'm remembering right, you don't absolutely need the bootcamp assistant to boot back into Windows. When you start your Mac up, you should see an option to boot into the OS X partition or the Windows partition.
If you did an upgrade install of Windows 8.1, it will still be installed. Check in the Control Panel and it should have it's own section there.
I haven't done this since the Snow Leopard days so forgive me if my information isn't truly correct.