Existing projects will be fine going from VS2013 to VS2015.
>You won't likely have the Business layer generator in VS 2015 but the projects should work fine. Last I heard OakLeaf was working on a update for VS 2015. I have several projects that I work within VS 2015 without issues but I also do not use the assemblies from the GAC.
Craig Berntson
MCSD, Microsoft .Net MVP, Grape City Community Influencer