When you open the project in SSDT 2015, the project will go through a one-way upgrade. That's about it then, unless you have any custom tools on SSRS/SSIS, where you'll need to source a SSDT 2015 version for your addon.
Just make sure that when you're deploying the project/packages to your server, you have to correct destination server version selected.
Thom~
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.
Larnu.uk