I am using a Mackbook Pro that I purchased in 2011 using OS X 10.8.5 and want to upgrade it to Yosemite I want to upgrade to Yosemite but before I do, I need to know if the upgrading process will require I reinstall all my apps once the upgrade is done. I am in the middle of a lot of work and don't have time to re-install every app again such as Photoshop, Lightroom and a host of other phoography related software.

Thanks for your help.

Charles
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.