From NUnit 2.x

In general, there are two approaches to running tests built against earlier versions of the NUnit framework from 2.0 through 2.5.10:

  1. Run the tests without recompilation. This is the best approach when no further changes are to be made to a project. Ensure that the framework assembly against which the tests were built continues to be avaialble in the test directory.
  2. Recompile the tests referencing the latest version of the framework assembly. This is usually the best option when a project is still under development.

Note: NUnit 2.6 no longer recognizes "old style" test cases - those identified by a name beginning with "Test". If you have such tests, you will need to convert them to use the TestAttribute and recompile your tests. In such cases, it makes sense to recompile against the latest framework.

From NUnit 1.x

Tests originally compiled against a version of NUnit prior to 2.0 will need to be rewritten and recompiled in order to run under NUnit 2.6.