Prior to Visual Studio 2013, SQL Server Data Tools (SSDT) was a stand alone application that you could install in the VS2010 or VS2012 shell. Now it’s one of the optional extras in the install of VS2013.
I’ve installed VS2013 with the SSDT optional feature and opened an existing database solution that was originally created in VS2012. The solution opens fine and there is no upgrade required by the looks of it. At first glance it looks like it will all just work okay.
I was then able to reopen the solution in VS2012 and that still seemed to work okay as well so it looks likes you can use SSDT database solutions in both VS2012 and VS2013 without any conversions or upgrades and you can pick and choose which version you want to use.