From NUnit 2.x
Beginning with version 2.2.1, old style test cases ("Test....") are no longer recognized by default. We recommend that you convert such test cases to use the TestAttribute. Alternatively, you may specify a setting in the test config file to allow use of old style test cases by default.
Beginning with NUnit 2.2.2, NUnit is able to run tests Built with older versions of NUnit 2.x without recompilation. Note that you must have an available copy of the nunit.framework assembly from the older version in order for your tests to load correctly.
From NUnit 1.x
NUnit 2.5 no longer supports inheriting from TestCase when defining a test. If you need to run such tests, you may continue to do so using the a 2.4.x or earlier version of the nunit.framework assembly. Of course, you will not be able to use new features introduced in 2.5 if you follow this course.
For a complete conversion to 2.5, you should modify and recompile your tests using the new version of NUnit.
Suite property
The NUnit 1.x Suite property will not be found by the new program. These must be changed to the "Suite" attribute for the test runners to find them. Another alternative is that these suites are no longer needed due to the automatic capability that is built in to the new version.
AssertionFailedError
If you have written code expecting the exception AssertionFailedError, this must be changed to AssertionException.