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