I am using a Mackbook Pro that I purchased in 2011 using OS X 10.8.5 and want to upgrade it to Yosemite
No, it won't - Since at least Snow Leopard, the upgrade is just done over the top of the original install. The core of the system doesn't change much these days, it's more about adding features/apps.

I'd recommend backing your work up anyway (even if you don't upgrade!) and if you have an urgent deadline, upgrade afterwards... base the time on the assumption you'll have to re-install, simply because installations can and do (fairly rarely) go wrong.