I can open Vivado 2015.1 projects in Vivado 2015.2 without a project upgrade.
I can also open a Vivado 2015.2 project in Vivado 2015.1 without getting the following "Future version" message:
Is this expected behavior?
In most releases Xilinx will update the project version.
The most common reason for this is that there is a change (for example, a new project property) requested for the project file (xpr) structure.
In the case of Vivado 2015.1 and 2015.2, the project files both contain the following version string.
To verify which version a project was created with, use the Product Version string.
Product Version: Vivado v2015.2 (64-bit)