1. A method of operating a computer system to validate test cases for test models comprising operating the computer system to perform steps of: receiving information for a first state transition diagram representative of a first test model that models a system under test (SUT); generating first test cases from the first test model using a test generator, each test case comprising a plurality of states in the first state transition diagram and a plurality of transitions between the states; receiving information for a second state transition diagram representative of a second test model that models a new version of the SUT; and for each test case among some of the first test cases: generating a third test model corresponding to said each test case using information for the second test model, including modifying a copy of the second test model by removing edges in the copy of the second test model that are not in said each test case, and copying and inserting edges and vertices that are traversed more than once in order to obtain the same test sequence as in said each test case; and designating said each test case as being valid or invalid for the second test model depending on whether or not the test generator can generate any test cases from the third test model.