How do I re-install Mavericks on rMBP without wiping drive?

I recently updated my 2014 Retina MacBook Pro to Yosemite, and there are major video issues.

I want to do a clean install of Mavericks, but I don't want to erase the drive, as there's nearly 1TB of media & documents on it.

Is this possible? Do I need to make a boot USB stick or something?

Or boot off an external? (I have a CCC pre-Yosemite backup on an external)