A project created or opened in Project Navigator 11.2 (or any version of ISE 11.x) should be openable in any other ISE software 11.x version without needing to be migrated or converted.
However, if the device family used in the project is not available in the install version that you open the project with, the following dialog message appears:
If all of the following circumstances are true, the .XISE file will become corrupted and will no longer be openable in any version of ISE:
If the circumstances listed above are all true and you select "No" so that the project will not be opened, the .xise file is overwritten with an empty one.
As a result, if you later try to open the project with any version of ISE software, the project will not be opened.
This issue has been fixed in ISE 11.2.
However, because ISE 11.1 has already been released, it is still possible to see the issue if you are working with both ISE 11.1 and a later version.